Trunked radio repeater system

ABSTRACT

A digital trunked radio repeater system provides substantial improvements in timeliness of channel acquisition and channel drop, and in reliability of critical control signalling. The system uses a much higher digital signalling rate than is typically found in prior art systems, and uses a control channel to convey digital channel request and assignment messages between the central site and mobile transceivers. The mobile radio transceivers transmit channel requests on the control channel (if no response is received, the mobile retries during a retry time window which increases in duration in dependence on the number of retries). The mobile transceiver switches to a working channel in response to an assignment message received on the control channel. Subaudible digital signals transmitted on the control channel and on active working channels allow late entry, shifting to higher priority calls, and other advanced functions. Message and transmission trunking capabilities are both present so as to maximize working channel usage without compromising channel access for high priority communications. During transmission trunking, called and calling transceivers return to the control channel after each transmission (and called transceivers may be inhibited from transmitting) but grant higher priority to calls from the other transceivers being communicated with to ensure continuity over an entire conversation. Additional functions and fault tolerant features further increase the versatility and reliability of the system.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of commonly-assigned copending application Ser. No. 056,922 of Childress et al filed June 3, 1987 (now U.S. Pat. No. 4,905,032) entitled "Trunked Radio Repeater System." The entire disclosure of that patent application is expressly incorporated herein by reference.

This invention is generally directed to the art of trunked radio repeater systems. It is more particularly directed to such systems using digital control signals transmitted over a dedicated control channel while also using plural working channels which are assigned temporarily for use by individual radio units.

The trunking of radio repeaters is well-known. Early trunking systems used analog control signals while some more recent systems have utilized digital control signals. Control signals have been utilized on a dedicated control channel and/or on different ones of the working channels for various different reasons and effects. A non-exhaustive but somewhat representative sampling of prior art publications and patents describing typical prior art trunked radio repeater systems is identified below:

U.S. Pat. No. 3,292,178--Magnuski (1966)

U.S. Pat. No. 3,458,664--Adlhoch et al (1969)

U.S. Pat. No. 3,571,519--Tsimbidis (1971)

U.S. Pat. No. 3,696,210--Peterson et al (1972)

U.S. Pat. No. 3,906,166--Cooper et al (1975)

U.S. Pat. No. 3,936,616--DiGianfilippo (1976)

U.S. Pat. No. 3,970,801--Ross et al (1976)

U.S. Pat. No. 4,001,693--Stackhouse et al (1977)

U.S. Pat. No. 4,010,327--Kobrinetz et al (1977)

U.S. Pat. No. 4,012,597--Lynk, Jr. et al (1977)

U.S. Pat. No. 4,022,973--Stackhouse et al (1977)

U.S. Pat. No. 4,027,243--Stackhouse et al (1977)

U.S. Pat. No. 4,029,901--Campbell (1977)

U.S. Pat. No. 4,128,740--Graziano (1978)

U.S. Pat. No. 4,131,849--Freeburg et al (1978)

U.S. Pat. No. 4,184,118--Cannalte et al (1980)

U.S. Pat. No. 4,231,114--Dolikian (1980)

U.S. Pat. No. 4,309,772--Kloker et al (1982)

U.S. Pat. No. 4,312,070--Coombes et al (1982)

U.S. Pat. No. 4,312,074--Pautler et al (1982)

U.S. Pat. No. 4,326,264--Cohen et al (1982)

U.S. Pat. No. 4,339,823--Predina et al (1982)

U.S. Pat. No. 4,347,625--Williams (1982)

U.S. Pat. No. 4,360,927--Bowen et al (1982)

U.S. Pat. No. 4,400,585--Kamen et al (1982)

U.S. Pat. No. 4,409,687--Berti et al (1983)

U.S. Pat. No. 4,430,742--Milleker et al (1984)

U.S. Pat. No. 4,430,755--Nadir et al (1984)

U.S. Pat. No. 4,433,256--Dolikian (1984)

U.S. Pat. No. 4,450,573--Noble (1984)

U.S. Pat. No. 4,485,486--Webb et al (1984)

U.S. Pat. No. 4,578,815--Persinotti (1985)

Bowen et al is one example of prior art switched channel repeater systems which avoid using a dedicated control channel--in part by providing a handshake with the repeater site controller on a seized "idle" working channel before communication with the called unit(s) is permitted to proceed.

There are many actual and potential applications for trunked radio repeater systems. However, one of the more important applications is for public service trunked (PST) systems. For example, one metropolitan area may advantageously utilize a single system of trunked radio repeaters to provide efficient radio communications between individual radio units within many different agencies. Each agency may, in turn, achieve efficient communication between individual units of different fleets or sub-units (e.g., the police department may have a need to provide efficient communications between different units of its squad car force, different portable units assigned to foot patrolmen, different units of detectives or narcotics agents and the like). Sometimes it may be important to communicate simultaneously to predefined groups of units (e.g., all units, all the squad cars, all of the foot patrolmen, etc.). At the same time, other agencies (e.g., the fire department, the transportation department, the water department, the emergency/rescue services, etc.) may be in need of similar communication services. As is well-known to those familiar with trunking theory, a relatively small number of radio repeaters can efficiently service all of these needs within a given geographic area if they are trunked (i.e., shared on an "as-needed" basis between all potential units).

This invention also is especially adapted for special mobile radio (SMR) trunked users. Here, an entrepreneur may provide a trunked radio repeater system at one or more sites within a given geographic area and then sell air time to many different independent businesses or other entities having the need to provide efficient radio communication between individual units of their particular organization. In many respects, the requirements of an SMR user are similar to those of a PST user.

In fact, the potential advantages of trunked radio repeater systems for public services is so well recognized that an organization known as the Association of Public-Safety Communications Officers, Inc. (formerly the Association of Police Communications Officers) (APCO) has developed a set of highly desirable features for such a system commonly known as the "APCO-16 Requirements." A complete listing and explanation of such requirements may be found in available publications known to those in the art.

One of the APCO-16 requirements is that any user must have voice channel access within one-half second after engaging a push-to-talk (PTT) switch. This same requirement must especially be met in emergency situations--and that implies that the system must be able to actively drop lower priority users also within a very short time frame. And, of course, the ability to quickly and efficiently drop channel assignments as soon as channel usage is terminated is also important for efficient usage of the trunked facility even in non-emergency situations.

Prior trunked radio systems have attempted to more or less "just meet" such APCO-16 requirements of timeliness. For example, published specifications of one such prior system indicates an ability to achieve channel update (in a 19 channel system) within 450 milliseconds and channel drops within 500 milliseconds. To achieve this, it utilizes 3,600 bits per second (bps) digital signalling over a dedicated digital control channel. Unfortunately, although theoretically the APCO-16 requirements of timeliness should be met by such a prior system, in reality, the APCO-16 timeliness requirements are often not met--or, are met only at the expense of suffering with the obviously adverse effects of somewhat unreliable digital control signalling (which are, at best, annoying even in non-emergency situations). Accordingly, there is considerable room for improvement.

The present invention provides substantial improvements--both in timeliness and in reliability of critical control signalling in a digitally trunked radio system of this general type. To begin with, a much higher digital signalling rate (9600 bps) is utilized. However, rather than using all of the increased signalling rate to provide a 9600/3600=2.67 improvement factor in timeliness, a large portion of the increased signalling rate capacity is utilized to improve signalling reliability. Accordingly, the increased timeliness of 19 channel updating capability, for example, is improved by a factor of approximately 1.58 (e.g., 285 milliseconds versus 450 milliseconds) while the rest of the increased signalling capacity is utilized to increase the reliability of control signalling. At the same time, virtually all of the increased signalling capacity is utilized to improve the timeliness of channel drop ability (e.g., 190 milliseconds versus 500 milliseconds).

As previously demonstrated by Bell System Technical Journal articles on the AMPS system (e.g. "Voice and Data Transmission", by Arredondo et al, The Bell System Technical Journal, Vol. 58, No. 1, January 1979, pp 97-122), digital data rates on radio channels should be either very low (e.g., 200 hertz) or as high as the channel bandwidth permits. The present invention utilizes the maximum high speed data rates (e.g., 9600 bps on the typical 25 KHz bandwidth radio channel) for critical control channel signalling and control signalling on the working channels both immediately before and immediately after the user communication interval. In addition, subaudible low-speed digital data is also utilized on the working channel during user communications so as to assure additional signalling reliability--and to also permit implementation of additional features. Bursts of high-speed data are occasionally inserted into the subaudible low-speed digital data stream transmitted on the working channels to further increase signalling capacity.

In the exemplary embodiment, all channels (the control as well as working channels) are fully duplexed so that there may be simultaneous inbound and outbound signalling on all channels. In general, this invention achieves reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units as specified by digital control signals on the control channel.

Channel assignment is initially requested by a calling radio unit passing digital request signals to a control site over the active control channel. In accordance with channel availability, a controller at the central site assigns a specific then-available working channel to the requested communication and passes digital assignment signals outbound over the control channel. Both the calling radio unit and the called unit(s) detect the working channel assignment and switch their transmitter and receiver operations over to the proper working channel. Thereafter, digital handshake signals are again exchanged between the control site and at least one of the radio units (e.g., the calling unit) over the assigned working channel. In response to a successful handshake on the assigned working channel. the central site then transmits digital release signals over the assigned working channel so as to release the appropriate units for communication thereover.

As one technique for increasing reliability, the initial request signals may include three-fold data redundancy (at least for critical data) while the channel assignment signals subsequently transmitted over the control channel may include as much as six fold redundancy of data (e.g., at least of critical data such as that representing the called party and the assigned channel). The handshake signals subsequently exchanged on the assigned working channel may also include three-fold data redundancy of at least critical data. In this manner, some of the increased signalling capacity made available by the high-speed data rate (e.g., 9600 bps) is sacrificed in favor of more reliable channel allocation and communication functions--while still comfortably exceeding all APCO-16 requirements.

To insure responsiveness to higher priority calls, subaudible digital channel assignment update messages are also transmitted over the assigned working channel. These are monitored in each unit then residing on the working channels. Accordingly, if a higher priority call is directed to some unit already engaged in a communication, that unit is enabled to promptly switch operations to a new assigned working channel so as to immediately receive the higher priority call.

In addition, to accommodate late entry of called parties to an ongoing communication, digital channel assignment "late entry" messages continue to be transmitted on the control channel even after a successful channel assignment process has been effected so that late entrants (e.g., those just turning on their radio, just passing out of a tunnel or from behind a building or otherwise back int radio communication after temporary interruption, completion of a higher or equal priority call, etc. ) may nevertheless be switched onto the proper assigned working channel as soon as possible. (The late entry feature, per se, is related to copending commonly assigned application Ser. No. 725,682 filed Apr. 22, 1985.)

To effect prompt and reliable termination of channel assignments, when the PTT switch of a calling unit is released, it sends a digital unkeyed message on the assigned working channel and, in response to reception of this unkeyed message at the control site, a digital drop signal is transmitted over the assigned working channel so as to immediately drop all units therefrom and thus free that working channel for reassignment. (As will be appreciated, a given radio unit will automatically revert to monitoring the control channel upon dropping from an assigned working channel.)

The system of this invention is sometimes termed a "digitally" trunked system because trunking control is effected by digital signals passed over a continuously dedicated "control" data channel. All units are programmed so as to automatically revert to the predetermined primary control channel upon being turned on or being reset. If the expected control channel data format is not there discovered, then alternate possible control channels are successively monitored in a predetermined sequence until an active control channel is discovered. In this manner, the usual control channel apparatus at the central control site may be temporarily removed from service (e.g., for maintenance purposes). This same feature also permits continued trunked system operation in the event that the regular control channel unexpectedly malfunctions or is otherwise taken out of service.

The exemplary embodiment of this invention is designed in such a way that it meets and, in many areas, surpasses all of the existing APCO-16 requirements. It may also support available voice encryption techniques, mobile digital data terminals (digital data may be passed in lieu of analog voice data during a trunked radio communication session) and/or available automatic vehicular location systems. Preferably, a fault tolerant architecture is used [see related copending commonly assigned U.S. application Ser. No. 057,046 filed concurrently herewith] so as to maintain trunked system operation even if the central processor happens to fail at the control site. If digital data is to be communicated between radio units and/or the central site, then it may be processed through the system in a manner similar to analog voice signals. In particular, such digital data communication will be carried out at a rate accommodated within the existing audio pass band and will be trunked just like desired voice communications (i.e., no dedicated digital data communication channels are required). To help increase reliability of digital data communications, data transmissions (and analog voice transmissions, as well) may be voted in voting systems employing satellite receivers connected to the central control site.

In the exemplary embodiment, digital control signalling messages of the following types are utilized:

    ______________________________________                                         Channel Type                                                                              Direction           Rate                                            ______________________________________                                         Control Channel                                                                           INBOUND             9600 pbs                                                   Group Call                                                                     Special Call                                                                   Status                                                                         Status Request/Page                                                            Emergency Alert/Group Call                                                     Individual Call                                                                Cancel Dynamic Regroup                                                         Dynamic Regroup - Forced                                                       Select                                                                         Dynamic Regroup - Option                                                       Deselect                                                                       Dynamic Regroup - Option                                                       Select                                                                         Login/Dynamic Regroup                                                          Acknowledge                                                                    Logical ID Request                                                             Programming Request                                                            OUTBOUND            9600 bps                                                   Channel Assignment                                                             Channel Update                                                                 Enable/Disable Unit                                                            Dynamic Regroup                                                                Preconfiguration                                                               Alias I D                                                                      Unit Keyed/Unkeyed                                                             Emergency Channel Assignment                                                   Cancel Dynamic Regroup                                                         Dynamic Regroup - Forced                                                       Select                                                                         Dynamic Regroup - Optional                                                     Select                                                                         Dynamic Regroup - Optional                                                     Select                                                                         Assign Group ID An Alias ID                                                    Assign Logical ID An Alias ID                                                  Status Acknowledge/Page                                                        Time Mark                                                                      Emergency Channel Update                                                       Site ID                                                                        System Operationa Made                                                         Site Status                                                                    Logical ID Assignment                                                          Programming Channel Assign-                                                    ment                                                                Working Channel                                                                           INBOUND             9600 bps                                                   Initial Handshake                                                              Special Call Signalling                                                        Unit ID-PTT and Reverse PTT                                                    Miscellaneous                                                                  OUTBOUND            9600 bps                                                   Initial handshake                                                              Channel Drop                                                                   Status Messages                                                                INBOUND             Low Speed                                                  Confirm Unit PTT                                                               OUTBOUND            Low Speed                                                  Priority Scan                                                                  Falsing Prevention                                                  ______________________________________                                    

Some of the general features of the exemplary embodiment and expected benefits are summarized below:

    ______________________________________                                         FEATURE            BENEFIT                                                     ______________________________________                                         VERY SHORT AVERAGE Practically instantaneous                                   CHANNEL ACCESS TIME                                                                               access doesn't cut off                                                         syllables.                                                  Normal Signal Strength                                                                            Provides operation which                                    Areas: 280 Milliseconds                                                                           is faster than most coded                                   Weak Signal Areas (12dB                                                                           squelch systems.                                            Sinad: 500 Milliseconds                                                        LATE ENTRY         Minimizes missed                                            Should a mobile turn on                                                                           conversations, keeps                                        during the period in which                                                                        police up to the minute,                                    its group is involved in                                                                          minimizes call backs.                                       a conversation, the mobile                                                     will automatically be                                                          directed to join that                                                          conversation.                                                                  AUTOMATIC CHANNEL  Frequency coordination                                      SWITCHING          of the fleets and                                           Mobiles, portables, control                                                                       subfleets requires                                          stations, and consoles                                                                            no action on the part of                                    automatically switch to                                                                           any field personnel.                                        the appropriate channel.                                                       HIGH SPEED CALL PRO-                                                                              Dedicated control channel                                   CESSING            provides more rapid                                         Processor assigns unit                                                                            channel assignments on                                      initiating the call, and                                                                          larger systems. System                                      all called units, to an                                                                           size does not impact                                        appropriate working channel.                                                                      upon channel acquisition                                    Initial channel assignment                                                                        time. Control channel is                                    communication between site                                                                        available for additional                                    controller and radio units                                                                        functions such as status                                    occurs on the control                                                                             and unit ID.                                                channel.                                                                       CALL RETRY         Eliminates the need for                                     Calling unit will  repetitive PTT operations                                   automatically repeat its                                                                          by the operator in weak                                     request up to eight times                                                                         signal situations.                                          if no response is received.                                                                       Terminating retries also                                    Retries terminated upon                                                                           shortens the signalling                                     system response.   time.                                                       UNIT DISABLE       In hostage situations,                                      Trunked units can be                                                                              these units can be                                          disabled on an individual                                                                         assigned to a special                                       basis. These disabled units                                                                       group for communication                                     continue to monitor the                                                                           with the criminals.                                         control channel and can be                                                                        Also, with automatic                                        polled to determine their                                                                         vehicular location,                                         status.            these units could be                                                           tracked for apprehension.                                   SUPPLEMENTARY CONTROL                                                                             Provides user and system                                    CHANNEL FUNCTIONALITY                                                                             operator with system                                        In addition to providing                                                                          manager features not                                        channel assignments, the                                                                          available on other types                                    control channel is used                                                                           of systems.                                                 for: status messages,                                                          polling, system status,                                                        logging, late entry,                                                           dynamic regrouping,                                                            system testing and other                                                       system functions.                                                              GROUP PRIVACY      Each group has the same                                     Each group hears only his                                                                         privacy as having their                                     own group, unless  own channel with the                                        specifically programmed                                                                           additional benefits of                                      otherwise. Dispatcher                                                                             being regrouped with                                        can override for individual                                                                       other complementing                                         units or groups of mobiles                                                                        functions for emergency                                     at any time.       operations.                                                 CALL QUEUING       Maintains orderly entry                                     When all channels are                                                                             procedure for busy                                          busy, call requests will                                                                          system. Call requests                                       be queued until a channel                                                                         are accepted in the order                                   becomes available. Unit                                                                           they are received except                                    requesting a channel will                                                                         higher priority users go                                    be notified to prevent                                                                            to the head of the line.                                    call backs. Members of                                                         groups already in the queue                                                    will not be reentered in                                                       the queue.                                                                     DATA COMMUNICATIONS                                                                               This feature greatly                                        System has the optional                                                                           enhances the value of the                                   capability of using 9600                                                                          system because it avoids                                    bps data on the working                                                                           the expense of additional                                   channels. Data     RF channels.                                                communications will take                                                       place on any equipped                                                          working channel and they                                                       are trunked, just as                                                           voice communication.                                                           VOICE ENCRYPTION   Voice encryption offers                                     System has the optional                                                                           the same encrypted range                                    capability of using                                                                               as for clear voice                                          available 9600 baud voice                                                                         transmissions. Voice can                                    encryption. (See, e.g.                                                                            be passed from each site                                    commonly assigned U.S.                                                                            through conventional                                        Pat. No. 4,622,680 voice grade phone lines                                     and application Ser.                                                                              or microwave links. Only                                    Nos. 661,597 filed 17                                                                             minor modifications are                                     October 1984, 661,733                                                                             needed to the base                                          filed 17 October 1984                                                                             station interface                                           and 661,740 filed 17                                                                              equipment to accommodate                                    October 1984.)     such sophisticated                                                             voice security systems.                                                        Any mobile can be                                                              upgraded to this                                                               system. Any mobile can                                                         be upgraded to this                                                            capability by adding an                                                        external module. No                                                            internal changes to the                                                        radio are required.                                         UNIT IDENTIFICATION                                                                               Each unit on each                                           All units are automatically                                                                       transmission is                                             identified when they                                                                              identified by the same                                      transmit. This is true                                                                            ID regardless of the                                        regardless of whether the                                                                         agency, fleet or                                            transmission takes place                                                                          subfleet in which he is                                     on the control channel                                                                            currently operating.                                        or on a working channel.                                                                          16,384 is many times                                        The system is able to                                                                             more than the logical                                       accommodate 16,384 discreet                                                                       number of users in a                                        addresses independent of                                                                          fully loaded twenty                                         fleet and subfleet.                                                                               channel system so there                                                        is more than sufficient                                                        capacity.                                                   TRAFFIC LOGGING    Statistics on system                                        All system information is                                                                         usage, such as peak                                         logged. Each unit trans-                                                                          loading, individual and                                     mission causes the units                                                                          group usage versus time,                                    ID, agency, fleet, sub-                                                                           as well as many other                                       fleet, channel, time, site                                                                        system parameters, are                                      and list of sites involved                                                                        available for tabulation                                    to be logged for manage-                                                                          and analysis.                                               ment reports.                                                                  TELEPHONE INTERCONNECT                                                                            All mobiles and portables                                   Authorized mobiles have                                                                           in the system can be                                        the ability to place and                                                                          interconnected to the                                       receive calls and patch                                                                           telephone system. Those                                     them to individuals or                                                                            mobiles not specifically                                    groups of mobiles which                                                                           equipped for this can be                                    may or may not be equipped                                                                        patched by their dis-                                       for telephone interconnect.                                                                       patcher to maintain                                                            adequate control of                                                            system loading factors.                                     GROUP PRIORITY ASSIGN-                                                                            System manager can set                                      MENT               individual group                                            Eight priority levels are                                                                         priorities according to                                     provided in the system.                                                                           the criticality of their                                    Each group (as well as                                                                            service.                                                    each individual) is                                                                               The flow of traffic is                                      assigned a priority.                                                                              more easily maintained                                                         by providing recent users                                                      priority over nonrecent                                                        users of the same                                                              priority level.                                             SUBAUDIBLE SIGNALLING/PRIORITY SCAN                                            Subaudible signalling on                                                                          Mobiles and portables                                       working channels in use                                                                           can be immediately                                          updates called and calling                                                                        directed to higher                                          units with recent channel                                                                         priority                                                    assignments - allowing                                                                            communications while                                        them to change to a working                                                                       involved in a lower                                         channel carrying higher                                                                           priority communications                                     priority communications.                                                                          on another working                                          High-speed data bursts                                                                            channel.                                                    embedded in the subaudible                                                     signalling or ensure                                                           rapid receipt of priority                                                      information.                                                                   ______________________________________                                    

In the exemplary system, 11 bits are available to determine the address of a unit within an agency, fleet or subfleet. Fourteen bits are available to determine the individual identity of a particular unit (i.e., its "logical ID"). The use of 11 bits for determining group addresses within an agency, fleet or subfleet provides great flexibility such that each agency may have many different fleet and subfleet structures. Furthermore, unit identification is not limited by a particular fleet and subfleet structure. The 16,384 unit identification codes can be divided among the subfleets in a manner that best suits a particular system.

Some features of this exemplary system which are believed to be particularly unique and advantageous are summarized briefly below (order of appearance not reflecting any order of importance--nor is this list to be considered in any way exhaustive or limiting):

(a) Widening the Retry Window

If a requested working channel assignment is not achieved, the request is automatically retried--and the time window in which such a retry is attempted is increased in duration as a function of the number of prior unsuccessful retries. This significantly decreases the average channel access time where noise is the real problem rather than request collisions--while still providing a recovery mechanism for request collision problems as well.

(b) Better Use of Subaudible Signalling

Because the data rate of subaudible signalling on the working channels is low compared to the high-speed signalling occurring on the control (and working) channels, subaudible messages may be substantially delayed relative to corresponding high-speed signals. For this reason, subaudible signalling may sometimes be unreliable in reporting or confirming events which occurred more than two seconds earlier--and subaudible signalling may not effectively convey messages to mobile units rapidly enough for them to react to real-time events (e.g., a higher priority transmission trunked call on another working channel). In the exemplary embodiment, subaudible signalling reports most events (e.g., channel assignments) only a limited number of times (e.g., twice) and for only a limited time period to prevent outdated information from being reported. Additionally, windows are defined in the subaudible signalling stream which are reserved for bursts of hiqh-speed signalling. The high-speed signalling bursts assure that mobile units operating on working channels receive important information very rapidly. The mobile units are programmed to temporarily mute during these high-speed bursts so that users are not disturbed by the noise which might otherwise be heard.

(c) Minimizing Priority Communique Fragmentations by Dynamically Altering Scan Functions

After initiating a priority call, a radio temporarily (e.g., for two seconds) disables the usual multiple group scan on the control channel--in favor of looking for the highly probable returned higher priority call. This reduces the possibility of getting diverted momentarily into an ongoing lower priority communique--and also perhaps missing a fragment of the next higher priority communique. A similar temporary (e.g., two seconds) scan preference (except for priority calls) for a just-previously involved call group also helps prevent fragmentation of non-priority communiques.

(d) Use of Transmission-Trunked Bit in Channel Assignment

The trunking system has two trunking modes:

(a) Transmission Trunked Mode in which the working channel is de-allocated as soon as the calling unit unkeys, and

(b) Message Trunked Mode in which the working channel is de-allocated "n" seconds following a unit's unkeying, unless another unit keys onto the channel within such "n" seconds. "n" is called the "hang time".

By dynamically insuring that both called and calling units "know" that a transmission-trunking mode is in effect, the calling unit may immediately revert to the control channel upon PTT release--thus immediately freeing the working channel for drop channel signalling from the control site. The called units can also be positively prevented from ever transmitting on the working channel-thus avoiding multiple keying of radio units on the working channel.

(e) Automatic Addressing of Immediately Returned Calls

Both the called and calling units/groups are identified in the initial channel assignment signalling. The called unit captures the calling unit ID and is enabled to automatically address a return call to the just calling radio if the PTT switch is depressed within a predetermined period (e.g., 5 seconds) after the just completed communique even if the system is in the Transmission Trunked Mode. Not only does this simplify the necessary call back procedures and minimize access times, by allowing greater application of the Transmission Trunked Mode it also increases the probability of successful message exchanges--especially in poor signalling areas.

(f) 9600 bps Permits "Loose" Synchronization

Use of higher rate 9600 bps signalling permits simplified bit synchronization to be rapidly achieved by simple "dotting" sequences (i.e., a string of alternating ones and zeros 101010 . . . ). Thus, there is no need to keep information transfers precisely synchronized across all channels. This not only reduces hardware requirements system-wide, it also facilitates a more fault tolerant architecture at the control site.

(g) Improved Channel Drop Signalling

The drop channel signalling is simply an extended dotting sequence. Therefore, each radio may easily simultaneously look for drop channel signalling and channel assignment confirmations. This means that the control site may more immediately consider a given working channel available for reassignment--and, if "loaded up," immediately interrupt the drop-channel signalling to issue fresh channel assignment confirmation signals on the working channel (which each individual radio will ignore unless properly addressed to it). As a result, a "loaded" system (i.e., one where existing channel requests are already queued) may drop a working channel within about only 100 msec--and immediately reassign it to a queued request. Radios that happen to enter late into the call being dropped can detect that fact and properly drop from the channel because of the ability to simultaneously look for drop channel signalling and channel assignment confirmation signalling.

(h) Feature Programming

To avoid cumbersome feature programming (and reprogramming to add features) by factory or distribution personnel, novel procedures are employed which safely permit the end user to perform all such "programming." All units are programmed at the factory to perform all available functions. A function enable bit map and a unique physical ID are together encrypted at the factory and provided to the user as "Program Codes." When the user programs each device, its encrypted "Program Codes" are input to a Radio Programmer which, in turn, properly sets the feature enable bit map in a connected radio unit--and the decoded physical ID--and a "Just Programmed" bit). The "just programmed" radio device logs into the central controller with a request for a logical ID--based on its apparent physical ID. If illegal copying of function enabling Program Codes occurs, then the same logical ID will be assigned--and the usefulness of the radio within the trunked repeater system will be diminished.

(i) Double Channel Assignment Handshake--One Being on the Assigned Working Channel

A first 9600 bps channel assignment signalling exchange occurs on the control channel. However, a confirmation (i.e., a second handshake) then occurs on the assigned working channel. Thus, it is assured that the desired channel has been successfully assigned and locked onto before the central controller unmutes the called units on the assigned channel. To decrease system access time even further, the calling unit does not wait to receive and decode the confirmation on the assigned working channel before transmitting its key message (which is preceded by dotting). That is, the calling unit begins transmitting synchronization signals (dotting) on the inbound working channel at the same time the called units are decoding the information message transmitted by the site on the outbound working channel. This reduces system access time by as much as 40 ms, and reliability is not adversely affected because of redundancy in the fields of the channel assignment message on the control channel.

These as well as other objects and advantages of this invention will be more completely understood and appreciated by carefully studying the following detailed description of the presently preferred exemplary embodiment taken in conjunction with the accompanying drawings of which:

FIG. 1 is a general explanatory diagram of trunked radio repeater system in accordance with this invention;

FIG. 2 is a simplified block diagram of a central control site (as well as a satellite receiver site) in the trunked repeater system of FIG. 1;

FIG. 3 is a simplified block diagram of the general site architecture of the main controller for the central control site;

FIG. 4 is a simplified block diagram of the channel architecture used within each channel of the central site architecture shown in FIG. 3;

FIG. 5 is a simplified general block diagram of a technical mobile/portable radio unit to be utilized for communication within the trunked repeater system of FIG. 1;

FIG. 6 is a simplified flowchart of typical call processing sequence in the exemplary embodiment from the perspective of a calling unit;

FIG. 7 is a simplified flowchart of a call processing sequence within a called unit;

FIG. 8 illustrates the trunked channel dropping procedure and typical required drop times;

FIG. 9 generally illustrates call initiation signalling within the exemplary system as well as typical timing requirements;

FIG. 10 is a graphical illustration of the control signalling protocol utilized to initiate and terminate trunked radio communications on either an individual or group basis in the exemplary system;

FIGS. 11A and 11B are together is a simplified flowchart of suitable computer programs which might be utilized at the site controller, the calling unit and the called unit so as to achieve the signalling protocol of FIG. 10; and

FIG. 12 is a graphical illustration of subaudible control signalling protocol transmitted on active working channels in the exemplary system.

An exemplary trunked radio repeater system in accordance with this invention is generally depicted at FIG. 1. As illustrated, individual units of various groups communicate with each other (both within and possibly outside of their own group) via shared radio repeater channels located at a trunked repeater control site 100. The dispatch console 102 may be housed directly at the repeater station site 104 or may be remotely located via other communication facilities 106 as will be appreciated by those in the art. There also may be multiple dispatch consoles 102 e.g., one for each separate fleet) and a master or supervisory dispatch console for the entire system as will also be appreciated by those in the art.

The central site is depicted in somewhat more detail at FIG. 2 in conjunction with one or more satellite receiver sites 100-1. As will be appreciated, the satellite receiver sites are displaced spatially from the central site 100 such that radio reception may temporarily be better at one or the other of the chosen antenna sites. Thus, received signals from the satellite sites as well as the central sites are combined in "voter" circuitry so as to choose the best available signal for control or communication processes.

At the central site, a transmitting antenna 200 and receiving antenna 202 (which may sometimes be a common antenna structure) may be utilized with conventional signal combining/decombining circuits 204, 206 as will be apparent to those in the art. The transmitting and receiving RF antenna circuitry 200-206 thus individually services a plurality of duplex RF channel transmit/receive circuits included in a plurality of RF repeater "stations" 300, 302, 304, 306, etc. Typically, there may be 20 such stations. Each station transmitter and receiver circuitry is typically controlled by a dedicated control shelf CS (e.g., a microprocessor-based control circuit) as is also generally depicted in FIG. 2. Such control shelf logic circuits associated with each station are, in turn, controlled by trunking cards TC (e.g., further microprocessor-based logic control circuits) 400, 402, 404 and 406. All of the trunking cards 400-406 communicate with one another and/or with a primary site controller 410 via control data bus 412. The primary site controller (and optional backup controllers if desired) may be a commercially available general purpose processor (e.g., a PDF 11/73 processor with 18 MHz-J11 chip set) Although the major "intelligence" and control capability for the entire system resides within controller 410, alternate backup or "fail soft" control functions may also be incorporated within the trunking cards 400-406 so as to provide continued trunked repeater service even in the event that controller 410 malfunctions or is otherwise taken out of service. (More detail on such fail soft features may be found in commonly assigned application Ser. No. 057,045 filed June 3, 1987 entitled "Fail Soft Architecture For Public Trunking System.")

An optional telephone interconnect 414 may also be provided to the public switched telephone network. Typically, a system manager terminal, printer, etc., 416 is also provided for overall system management and control (together with one or more dispatcher consoles 102). A special test and alarming facility 418 may also be provided if desired.

The signal "voter" circuits 502, 504, 506 and 508 are connected so as to receive a plurality of input digital or analog signals and to selectively output therefrom the strongest and/or otherwise most reliable one of the input signals. Thus, received signals from the central site 100 are input to respective ones of the channel voter circuits 502-508 while additional similar input signals are generated from receivers in the satellite receiver site 100-1 and also input to the appropriate respective voter circuits. The results of the voting process are then passed back to the trunking card circuits 400-406 where they are further processed as the valid "received" signals.

A slightly more detailed view of the site architecture for control data communication is shown in FIG. 3. Here, the PDI 11/73 controller 410 is seen to communicate over a 19.2 Kilobit link 412 with up to 25 trunking control cards TC controlling respective duplex repeater circuits in those individual channels. Another high-speed 19.2 Kilobit link 420 is used to communicate with the hardware that supports the down link to/from the dispatch console 102. Other data communication with the central processor 410 is via 9600 baud links as shown in FIG. 3. The central processor 410 may include, for example, a 128 Kilobyte code PROM, 1 Megabyte of RAM and 32 DHV-11/J compatible RS-232C ports. It may typically be programmed using Micropower Pascal to provide a multi-tasking, event-driven operating system to manage all of the various data communication ports on an acceptable real time basis.

At each controlled repeater channel, the 19.2 Kilobit data bus 412 (as well as that from an optional backup controller if desired) is monitored by an 8031 processor in the TC module. The TC trunking control module exercises control over the control shelf CS of its associated repeater with audio, signalling, and control busses as depicted in FIG. 4, and may typically also receive hard-wired inputs providing clock synchronization and a "fail soft" indication (e.g., indicating that normal control by the central controller 410 is not available and that an alternate distributed control algorithm should then be implemented within each of the trunking control modules TC).

The general architecture of a suitable mobile/portable radio unit for use within the exemplary system is also microprocessor based as depicted in FIG. 5. Here, microprocessor 550 is provided with suitable memory 552 and input/output circuits 554 so as to interface with the radio unit display, keypad, push-to-talk (PTT) switch as well as audio circuits 556 which provide basic analog audio outputs to the speaker and accept analog audio inputs from the microphone. Auxiliary control over a modem 558 as a digital interface (e.g., to voice encryption, vehicle location or other types of digital communication subsystems) may also be provided if desired. And, of course, the I/O circuits 554 also permit suitable programmed control over RF receiver 560 and transmitter 562 which, via conventional signal combiners 564 permit two-way fully duplexed communication over a common antenna 566 as will be appreciated by those in the art.

A detailed and in-depth description of all units and sub-units of such a sophisticated system would necessarily be extremely voluminous and complex. However, since those in this art are already generally familiar with digitally controlled trunked repeater systems with suitable RF transmitter and receiver circuits, programmed general purpose computer controllers, etc., no such exorbitantly detailed description is believed necessary. Instead, it would only serve to obscure the subject matter which constitutes the invention. Accordingly, the remainder of this description will concentrate on the signalling protocol utilized to initiate and terminate calls within the system since this is believed to constitute a significant improvement (both in reliability and speed)--while still facilitating the retention of many highly desirable system features and meeting or exceeding all APCO-16 requirements.

Call placement begins by the calling unit transmitting a special digital channel request signal on the dedicated control channel to the central site. In return, the central site transmits, outbound on the control channel, a special digital channel assignment signal. The calling unit then responds by switching immediately to the assigned working channel. The central site sends an assignment confirmation message (also in high-speed digital form) over the assigned working channel to confirm to called units responding to the channel assignment signal that they are properly on the working channel (preventing a unit erroneously decoding a channel assignment message from unmuting for a working channel message not intended for it). Meanwhile, the calling unit does not wait for the confirmation message but instead immediately begins transmitting a synchronization signal. The synchronization signal allows the central site to confirm the calling unit has properly switched to the assigned working channel, in response to which the central site releases the called units to begin the requested communication session on the working channel (by transmitting a unit-keyed/unmute message in the exemplary embodiment). Alternatively, if during this process the calling unit receives a channel update message on the control channel addressed to it, then the channel request call is temporarily suspended (unless the channel request under way is an emergency or higher priority request) and the calling unit then reverts to the called state so as to receive the incoming call. If the calling unit receives no response (or an improperly completed response handshake sequence), it automatically waits a random period before retrying to successfully place the call request (up to a maximum of 8 tries).

The called unit initially resides in a standby configuration where it continually monitors the digital messages appearing on the control channel outbound from the central site. If it detects a channel assignment message addressed to it as the called party (or perhaps as one party of a called group), then the called unit immediately switches its operations onto the assigned working channel. There, it also detects the confirmation signals outbound on the working channel from the central site and, if successfully confirmed, awaits a release or unsquelching signal on the working channel (e.g., transmitted from the central site in response to successful completion of a handshake with the calling unit on the working channel). The called unit may also receive a channel update message indicating that the group is already operating on the correct channel.

The programming for a calling unit is generally depicted in the simplified flowchart of FIG. 6. Here, upon entry into the calling mode, a call request is sent on the control channel CC at step 600. A test is made at 602 for call queuing. If queued, transfer is made to wait loop 603 (including a test for a detected assignment at step 604 followed by a check for expiration of a 30 second timer at 606 (whereupon control effectively is passed back to a manual requirement to restart the calling process via exit 607).

If the call request is not queued, then a test is made at 608 to see if this particular unit has already previously been requested as a called party. If so, then transfer is made at 610 to the called mode of operation. If not, then a check for a returning channel assignment is made at 612. If not received at the expected time, then a random wait is interposed at 614 before a test is made at 616 to see if eight tries have yet been made to complete this particular call. If so, then the subroutine is exited at 618. If not, then the subroutine is re-entered at 600.

If a channel assignment is successfully detected at either 612 or 604, then the unit operation is immediately switched to the assigned working channel at step 620. The calling unit transmits an elongated sequence of dotting at 624 followed by a transmission of voice at 626 (or data if a digital communication session has been requested) over the assigned working channel before exit from the subroutine is taken at 628 (e.g., to a standard monitoring routine which looks for release of the PTT switch and transmits an unkeyed signal at 627).

The protocol followed by a called unit is generally depicted at FIG. 7 (e.g, representing a suitable computer program for controlling the unit in this mode of operation).

Upon entry, the control channel is simply monitored at 700 for any "good" message (e.g., one addressed to this particular unit). If such a message is detected, a check is made at 702 for an "update" type of message. If the message is of this type, then a check is made at 704 to see if it is repeated within about 1.0 seconds. If not, then reentry into the called mode is made. However, if the update of a higher priority incoming call is repeated within such period, then an immediate switch to the there-assigned working channel is made at 706. If signalling is not confirmed at 707, then an immediate switch to unsquelching (716) is made and that channel is thereafter monitored. If, on the other hand, signalling confirmation at 707 is achieved, it is an indication that normal channel assignment is, in reality, taking place and control is passed to block 714 to look for an unmute message.

If no channel update message is detected at 702, then the message is checked to see if it was a channel assignment at 708. If not, then return is made to the beginning of the subroutine. However, if a proper channel assignment has been received, then a switch to the assigned working channel is made at 710 and a check for proper confirmation signalling on the working channel is then made at 712. If a proper unmuting message is thereafter also received on that assigned working channel at 714, then the called unit unsquelches at 716. If no unmuting message is received at 714, then a check for a drop message is made at 718. If there is no drop message but high-speed signalling is still present on the working channel (as detected by 720), then a further check is made for the unmute message at 714. However, if there is no drop message and the high-speed signalling as ceased at 720, then the called unit is nevertheless unsquelched at 716.

At the conclusion of a desired audio call, the calling radio transmitter transmits a special release PTT signal as depicted graphically in FIG. 8. After a suitable transmission and detection delay period, the assigned working channel responds by transmitting a drop channel signal on the working channel. As shown in FIG. 8, this results in a typical working channel availability in only 167 milliseconds after the release PTT signal is initiated.

Typical timing of calling protocol signals is depicted graphically in FIG. 9 where it can be seen that typical calling protocol can be completed and communication begun over the desired working channel within about 245 milliseconds.

Some bit-level maps of some relevant message formats (and other related signalling formats and protocol) are graphically illustrated at FIG. 10. The control channel transmits an outbound continuous transmission repeating the format 800 depicted in FIG. 10. As will the seen, each 40 bit message is transmitted three times (including one inverted transmission where all 0's are changed to 1's and vice versa) and there are two such messages transmitted per recurring message time "slot." As will be appreciated, the optional dotting prefix (if used) insures continued bit synchronization by receiving units and the unique Barker code permits frame synchronization so as to define bit boundaries between the 40 bit-level messages which follow. Since the control channel transmits these message slots continuously, no dotting prefix is needed and but one transmission of the word framing Barker code will suffice for each recurrent transmission cycle. Of course, if desired, a relatively short dotting prefix may be used to even further insure continued bit synchronization.

Inbound messages on the control channel CC are of the format 802 shown in FIG. 10 and may comprise, for example, group/individual channel assignment requests transmitted from a calling unit. Here, the dotting prefix is considerably longer and the word framing Barker code is repeated three times so as to insure that the receiving circuits at the central site are properly synchrorized before the 40 bit messages (again with three-fold redundancy) are transmitted. Preferably, suitable transmission timing circuits are utilized so as to make such incoming control channel messages synchronously time "slotted"--meaning that the messages on the inbound portion of the control channel occur during the same time slot as outgoing messages from the central site on the control channel (as generally indicated by dotted lines in FIG. 10).

A group call request message format is shown in expanded scale in FIG. 10. It includes a 3-bit message type (MT) code (the message-type field may be extended in a tree-logic fashion to include additional bits as will be appreciated). This MT-A field thus distinguishes a group call from an individual call, for example, and may also indicate the type of communication session being requested. (If desired, a priority field of 1-bit also may be used to indicate if a highest priority emergency call is being requested.) The called identification code of 11 bits (representing either a group or an individual unit) is followed by a 14-bit field representing the identity of the calling unit ("logical ID"). The 40-bit message concludes with 12 bits of standard BCH code for error detection and correction purposes as will be appreciated.

The returning channel assignment message actually comprises a two message pair also having a format as shown in expanded scale at FIG. 10. The first three bits identify the message type (MT) and the type of communication session which is to take place. The identity of the calling unit is next represented by an eight-digit field, with the eight most significant bits being transmitted in one message of the two-message pair and the eight least significant bits being transmitted in the other of such messages (this information can be used to prevent collisions of radios calling the same group and might otherwise transmit on top of each other on the working channel). The next 5-bit field identifies the assigned working channel. This 5-bit field is followed by a 1-bit field indicating whether the call is transmission or message trunked--so that the calling unit may determine this trunking status before returning to the assigned working channel. The group or individual identity of the called unit(s) is contained within the next 11 bits followed by 12 bits of BCH error detection/correction code.

Once operation reverts to the assigned working channel, the central site transmits a confirmation message of format 804 outbound on the working channel. As will be observed, it is of the same general form as the continuous transmissions on the control channel CC except that the message length has been reduced to 32 bits on the working channel. Once again, the message is sent with three-fold redundancy (one being inverted). Preferably, the confirmation message is timed in the working channel so as to be within the same time slot as messages being transmitted on the control channel. The format of the 32 bit confirmation message is also depicted in expanded form at 806 in FIG. 10. Here, 4 bits are devoted to the message type (MT) code. A succeeding single bit indicates whether the communication session is transmission trunked or message trunked. Another bit of the confirmation message 806 identifies the call as being either of a group or an individual unit while the identity of the called group or individual unit is contained within the following 14 bits. The confirmation message 806 concludes with 12 bits of BCH error detection/correction code.

As soon as the calling unit has successfully tuned to the assigned working channel, the calling unit begins transmitting 384 bits of dotting followed by audio (in the case of a requested audio communication session) as is also depicted at FIG. 10.

The elongated dotted sequence transmitted by the calling unit on the working channel constitutes an acknowledgment of the successful handshake sequence and, in response, the central site transmits an outbound digital message on the working channel to positively unmute the called unit(s). The format 808 of such an unmute message is depicted in FIG. 10. Once again, the message type code uses the first four bits while the next bit is currently unused in the exemplary embodiment (but instead may be used for other optional purposes). The next bit denotes trunked status or non-trunked status (e.g., regular hang-time). The identity cf the unit(s) to be unmuted is set forth in the next 14 bits followed by 12 bits of standard BCH error detection/correction code.

At the conclusion of a communication session on the working channel, the calling unit again transmits 384 bits of dotting followed by 4 data blocks of 128 bits each. Each such data block includes 16 dotting bits and a 16 bit Barker code (some of which bits may be "filler" as will te appreciated) as prefix followed by 8-bit bytes, each of which is transmitted with three-fold redundancy (one inverted)--thus constituting a 32-bit message characteristic of digital messages being transmitted on the working channel. The format of the 32-bit unkey message 810 is also shown in FIG. 10. Here, a 4-bit message type code is followed by 2 bits for a block count (used to indicate repeats of the message in the data block). The identity of the calling unit is set forth in the next 14 bits followed by 12 bits of standard BCH error detection/correction code.

Finally, in response to receipt of the unkey message at the central site on the working channel, an outbound digital message on the working channel of a super-extended dotting sequence e.g., 896 to 2816 bits) is transmitted from the central site as depicted at 812 in FIG. 10--and in response, all units then on that channel drop from that particular working channel and revert to the active control channel.

The sequence of programmed events occurring at the site controller, the calling unit and the called unit(s) during a typical call origination/termination sequence is depicted in the parallel flowcharts of FIGS. 11A and 11B.

Each programmed unit has a quiescent control channel (CC) monitor routine where, in a quiescent state, all units and the site controller reside. When the calling unit enters the calling subroutine from the CC monitor at 1100, a test is made at 1102 to see if this calling attempt is a retry. If not, the retry counter is set to a maximum content of 8 at 1106 and then decremented by one at 1108 (which step is directly taken from test 1102 if a retry is in progress). If the retry counter has been decremented to zero as tested at 1110, then a failed acquisition audible beep is generated at 1112 and exit is taken back to the CC monitor. On the other hand, if the maximum number of retries have not yet been made, then a channel assignment request is transmitted on the control channel and slot synchronization at 1114 (e.g., at time t₁).

Upon detecting an inbound message, the site controller will receive and store the channel assignment request and assign a free working channel at step 1200. In the exemplary system, a response to an inbound request may be supplied within a predetermined delay. The outbound channel assignment messages (i.e., a message air) are transmitted on the control channel as soon as possible at step 1204 (time t₂). The two message channel assignment pair is then received and stored from the control channel in the calling unit at step 1118 (the unit will look for the messages up to the maximum number of slots). As previously explained, if a channel update is received in the interim, then an exit may be taken to a called state (assuming that the call request under way is not an emergency). If a valid channel assignment message has not been received as tested at 1120 and the maximum number of slots have been observed, then a suitable delay is loaded at 1122 and exit is taken back to the CC monitor (from which a return entry to the calling subroutine will soon be taken).

The process of loading in a suitable delay before retrying may be thought of as a progressive "widening" of the retry window--in a consciously controlled manner. There are three reasons why an inbound data message from a radio would not get a response: (1) the inbound message was not successfully detected; (2) the outbound message was not successfully detected; or (3) a collision occurred (two or more mobiles sent in a request on the same inbound control channel slot).

Given that a collision has occurred, unless mobiles randomly retransmit, their requests, collisions will continue to occur. Ccnsequently, when a radio fails to receive a response to an inbound message, it waits a "random" period of time to retransmit its request. However, if case (1) or (2) has occurred, there is really no reason to randomize the retry. Unfortunately, the radio cannot determine the cause of a failed response.

But, the longer a mobile waits to retransmit, the longer the average access time becomes in poor signalling areas since that is where the majority of retries take place. Since often it is noise and not collisions that cause missed responses, randomizing retries is often wasteful.

To correct this problem, the present invention takes some corrective action. First, non-channel-acquisition messages are caused to have a much slower retry rate than channel request messages. Access time for the former is not critical (whereas it is for the latter). So, if a collision occurs between a radio sending in a non-channel request message and a radio sending in a channel request message, the former's retry rate will be slow enough to guarantee no chance of a collision with the latter on the next retry.

Second, the random retry rate varies with the retry number. The retry algorithm (for channel acquisition messages only) widens the width of the retry window with each succeeding retry. This decreases the average access time in the presence of noise but still provides a recovery mechanism should the cause for a missed response be a collision.

The preferred embodiment uses the following simple rule:

    ______________________________________                                         1st retry         2 slot random variability                                    2nd retry         4 slot random variability                                    succeeding retries                                                                               8 slot random variability                                    ______________________________________                                    

It also is possible to vary the retry window width as a function of the received bit error rate in order to gain still greater efficiency.

If a valid working channel assignment has been received as tested at 1120, then the calling unit switches immediately to the assigned working channel at 1124 and begins to transmit 384 bits of dotting on the working channel at 1132 followed by voice transmissions (or other desired communication session) at 1134. Meanwhile, a confirmation message is transmitted on the working channel by the site controller at step 1206 at time t₃. The calling unit need not decode the confirmation message in the exemplary embodiment before transmitting its key message (block 132) because much of the information obtained in the two messages of the channel assignment message pair is redundant--and therefore the probability that the calling unit will "false" and go to an incorrect working channel is very small.

Back at the site controller, a check is made at 1208 for the acknowledgment dotting of extended duration on the working channel. If it is not received, then exit is taken. However, if it is properly received, then two unit-keyed/unmute messages are transmitted outbound on the working channel at step 1210.

While all of the above has been taking place, the called unit has (if everything is working properly) received and stored the two-message channel assignment pair from the control channel at time t₂ (at substantially the same time as the calling unit) as depicted at 1300. In response, the called unit is also switched to the assigned working channel at 1302 and has thereafter monitored the assigned working channel for the proper confirmation thereon at step 1304 (and at time t₃). Only if the proper confirmation message has been received does the called unit then look for and receive the unmute message transmitted from the site controller on the working channel at time t₅ and, in response, unmutes the receiver of the called unit on the working channel at 1306.

During the ensuing communication session on the assigned working channel between the calling and called unit(s), the site controller (via the TC's) continues to send subaudible new channel assignment (and drop) data to all units on all working channels at 1212 (thus enabling higher priority calls to be promptly received and accepted by all units). The site controller (via the proper TC) also continues to transmit channel update messages periodically on the control channel at 1214 (e.g., so as to permit late entrants to immediately go to the proper working channel). The site controller informs all TC's of the channel assignments and drops and, in response, each TC generates suitable subaudible signalling for its channel.

All subaudible signalling occurs at a baud rate of 150 bits per second in the exemplary embodiment--equivalent to a 75 Hz tone when dotting is being sent. Whenever a unit is transmitting in clear voice on a working channel, it transmits dotting for subaudible signalling. The central site uses the absence of this dotting in conjunction with key and unkey high-speed inbound working channel signalling to determine if a radio is transmitting on the channel.

The entire time a working channel is "on the air," a continual stream of subaudible data frames are transmitted by the central site (via the appropriate TC) to inform all units on the channel of other channel assignments taking place on the system. This mechanism is used to implement the priority scan feature--as will be explained shortly. A background process executed by the site controller continually sends channel update messages (via the proper TC) in channel order (non-busy channels are excluded). This process is interrupted anytime a new channel assignment is made, and the new channel assignment is transmitted at high priority (similar to the way a channel assignment interrupts the channel update message sequence on the control channel).

In the exemplary embodiment, subaudible channel updates are discarded and no longer transmitted after being transmitted twice (once at priority, once in background). This prevents the subaudible signalling from reporting stale information that is no longer accurate (e.g., channel assignments for a transmission-trunked message that has already terminated)--preventing units from switching to working channels that were but no longer are in use. By transmitting channel assignments via subaudible signalling for only a limited period of time (e.g., two seconds in the exemplary embodiment), stale information is prevented from propagating over the working channels.

Due to the high speed at which events occur in the system, the subaudible signalling data rate may be too slow to report events in time for a desired response to occur (e.g., move a called unit from one working channel to another in time to hear an emergency transmission). In the exemplary embodiment, this problem is overcome by providing pairs of bits in the subaudible signalling stream that are not actually used but are instead set aside for the bursting of high speed (9600 baud) data. This bursting is performed in a manner which prevents the high-speed data from corrupting the low speed data.

The subaudible signalling frame has a duration of 0.693 seconds and includes an 8-bit synch string (10111011 in the exemplary embodiment) followed by 96 bits of duplicated and interleaved data (see FIG. 12). The 96 bits of data actually consists of a 48-bit message in the exemplary embodiment--where every two bits are inverted and repeated (this maintains a steady DC point while effectively high pass filtering the signal for improved detection, and also increases the likelihood of successful decoding even when bit errors occur).

The use of the 48 bits is shown below:

    hhssi ggggggggggg hhssi ggggggggggg hhuu bbbbbbbbbbbb

Where:

h Bit space reserved for High-Speed Signalling (9600 baud)

s Spare Bit (defined as 0)

i Idle Frame Bit

g Group Id Bits

u Bit space reserved for High-Speed Confirmation

b BCH Code Bits

Note: h and u bits are defined to be 0 for BCH calculation.

Each frame contains two groups which are "on channel." If no groups are on channel, the frame is "idle" and has i set to 1 and the group field set to 7FF hex (all ones). If a single group is on channel, its id is placed in both slots of the frame.

Should an individual call or dynamic regroup take place, the high speed data will go out identically in all three high-speed slots of the subaudible frame.

Overflow from subaudible updates is transmitted via high speed signalling. Any of the three windows may be used. The message is not repeated (though all three windows may be used if subaudible is continuously overflowing).

For updating which group belongs on the channel, the special update high-speed slot is used in the first frame and in each eighth frame thereafter.

Note that the high-speed slot is located on even bit boundaries so that when the bits are duplicated and inverted as part of the overall subaudible transmission scheme, a 4-bit interval (26 milliseconds) is actually set aside for the high speed data.

The mobile units are preprogrammed to mute their audio during the 4-bit time high-speed slots to prevent high-speed signalling from generating noise in the audio heard by the user. In addition. the units are programmed to "force" these 2-bit values to preset values (0 is the exemplary embodiment) for purposes of error checking, and of course, to decode the high-speed signalling at the high speed rate when it comes in. The short high-speed signalling slots in the subaudible signalling frames do not interfere with background subaudible signalling and yet provide a mechanism by which the control site can very rapidly (e.g., at 9600 baud) transmit messages to units operating on working channels.

In existing systems subaudible signalling typically is used only as a validity check by mobiles. When a mobile is on a working channel it monitors the subaudible signalling to make sure it belongs on that channel. One reason why a radio might get onto a channel where it does not belong is that while monitoring the control channel, it incorrectly decodes a message and goes to an incorrect channel.

There is a sufficiently high probability of incorrectly decoding outbound control messages on existing systems that a quick way to redirect radios from channels where they do not belong is typically provided. To do this, subaudible signalling typically is used exclusively for this purpose. However, with this invention, advantage is taken of the high information rate on the control channel, and a mobile is required to see an update message twice before going to a working channel. There is a negligible increase in the late entry time, but the probability of going to an incorrect channel is virtually eliminated. As a result, subaudible data can also be used for another purpose . . . e.g., a priority scan--and high-speed data bursts provided in the subaudible data from in the exemplary embodiment ensure that priority information is received by affected units as soon as possible.

At the conclusion of the desired communications session, the unkeying of the PTT switch in the calling unit is detected at 1136 resulting in the sending of an unkeyed message on the working channel at 1138 (time t₆). If in a transmission-trunked mode, the calling unit may immediately revert to the control channel--thus immediately freeing the working channel (the calling unit was informed that the call was transmission trunked during receipt of the channel assignment message and does not wait to receive a responsive unkey signal from the site controller before reverting to the control channel). In response to receipt of the calling units' unkey message, at 1216, the site controller at 1218, sends a super elongated dotting string (896 to 2816 bits on the working channel at time t₇). The called unit has, of course, also received the unkeyed message on the working channel at time t₆ and, in response, has already muted the receiver at 1308. The called unit receives the super-elongated dotting string outbound from the site controller on the working channel at time t₇ and, in response, reverts to the control channel at 1310.

A special priority scan sequence is used (in the preferred embodiment) to minimize communique fragmentation.

When a radio unit scans for multiple groups and a call is made to its priority group, the radio automatically disables the multiple group scan (in favor of a priority group only scan) for a two second interval upon returning to the control channel. Since the priority group was just communicating, the probability is high that another communique will take place within this interval. If the radio immediately scanned into another (non-priority) group call (which by definition is of a lower priority), and another communique then occurs on the priority group, the radio would hear a communique fragment from a non-priority group--and would have its entry into the next priority group communique delayed (priority scan typically may take between 1.0 and 1.5 seconds to get the radio into the priority group).

Another unique feature used to minimize communique fragmentation is a preference priority the radio automatically assigns to a just-previously monitored non-priority group. In essence, if a non-priority communique is monitored, for two seconds following the communique the radio will ignore all other scanned calls (except to the priority group of course) . . . similarly to priority group communiques. In addition, the radio always remembers the last non-priority group monitored. Upon returning from a priority group communique, the radio will prefer the last non-priority group monitored over any other groups being scanned.

In the example below, a `--` means the group is involved in a communique channel, and it is assumed that Group A is the priority group and that its communiques are separated by less than 2 seconds: ##STR1##

There is a bit (i.e., the message/transmission trunked bit) in the working channel confirmation signalling that informs the radios as to whether the communique is transmission-trunked or message-trunked. This unique feature offers greater frequency efficiency.

The calling radio is guaranteed to see the message/transmission trunked bit when it receives the channel assignment message. If the bit is set to "Transmission Mode," the calling mobile knows the channel will be removed as soon as it stops transmitting. Consequently, when its PTT is released, the calling radio automatically and immediately goes back to the control channel. This gains channel usage efficiency because the working channel TC can begin channel drop signalling as soon as it detects the calling mobile's unkey message. That is, one does not have to extend the signalling to make sure the transmitting mobile finishes transmitting, gets its receiver on channel and then has plenty of time to be guaranteed that the channel drop signalling is detected in the calling mobile.

Radios that are called also look at this message/transmission trunked bit (in the channel assignment message and also in the working channel confirmation and unit keyed/unmute messages), but for an entirely different reason. If the communique is message-trunked, radios that were called must be able to key on the assigned working channel in case they must offer a response before the channel drop. However, if the communique is transmission-trunked, none of the called radios should ever transmit on the assigned working channel. Therefore, if the bit is set to "Transmission" mode, called radios will not be permitted to key on the working channel. This is a very useful feature since it prevents radios from keying on top of each other.

The message/transmission trunked bit thus offers three system advantages: It makes transmission trunking more frequency (i.e., channel) efficient by decreasing the channel drop time (by a factor of three from typical prior systems), it reduces the dead time between transmission where users can not key (e.g., on typical existing systems, if a radio is keyed during the 0.5 second drop sequence it must wait until the sequence is complete), and it offers absolute protection from radios keying on top of each other on a working channel.

To make an Individual Call on the exemplary system, the calling radio uses a single inbound slot of the control channel to identify itself and to specify the radio being called. Both radios are referred, via an outbound control channel message, to an available working channel where the confirmation signalling takes place. The unmute message to the called radio (at the completion of the high speed confirmation signalling) also specifies the ID of the calling radio. The called radio automatically stores the ID of the calling radio and, if the PTT switch of the called radio is depressed within 5 seconds of the last PTT release of the calling station, will automatically place an individual call back to the original calling. This capability allows easy user-convenient transmission trunking, and therefor better frequency (i.e., channel) efficiency, during individual calls. It also allows the calling radio to contact a called radio and converse with no channel hang time even though the called radio is not previously programmed to initiate a call to the calling radio.

The signalling in the exemplary embodiment is extremely efficient, minimizing the channel drop time and therefor increasing system efficiency. It is unique in that, for example, it is high speed signalling as opposed to low speed typically used on all other existing systems. In addition, the signalling is designed specifically for minimizing message traffic in a distributed architecture site.

Without such novel channel drop signalling, as the channel starts dropping, a message would have to be sent from the working channel TC to the control channel TC (via the site controller) stopping all updates on the outbound control channel (i.e., those that are referring radios to the working channel now being dropped). Once off the air, the channel TC would have to send an additional message to the site controller informing it of such so it can reassign the dropped working channel when appropriate. Besides additional messages within the central site slowing the channel drop process, such prior techniques also incur additional loading on the site controller. Another aspect of the problem is that the drop channel signalling that is transmitted on the working channel must be of sufficient duration to guarantee that timing ambiguities don't permit a radio to enter late onto the channel once it is down . . . or even worse to enter late onto the channel after the next call has already started to take place on that channel.

The exemplary embodiment uses unique drop channel signalling, a unique radio signalling detection algorithm and timing of when the channel TC sends the drop channel message to the site controller.

By making the drop channel signalling 9600 bps dotting, not only can the drop channel signalling be detected and muted in radios prior to the radio operator hearing the signalling, but the detection algorithm places a light enough processor loading on the radios that they can simultaneously look for the dotting and for confirmation signalling.

The following rules are followed by a working channel TC as it drops:

(1) Transmit 100 msec of dotting.

(2) Without interrupting the dotting, send a channel drop message to the site controller.

(3) Transmit an additional 200 msec of dotting . . . BUT . . . stop it and start sending a confirmation message should a channel assignment message be received from the site controller.

The following rules are followed by the site controller when it receives a drop channel message from a given channel TC:

(1) Immediately inform the control channel TC so it can stop transmitting updates to the working channel TC.

(2) Consider the channel immediately available for reassignment.

The following rules are followed by a radio as it leaves a working channel:

(1) For 1/2 second ignore all channel updates to the group and channel of the communique being left.

The following rules are followed by a radio as it arrives on a working channel:

(1) Look for dotting (i.e., of sufficiently long duration to constitute a drop channel signal) and confirmation signalling simultaneously.

(2) If drop-channel dotting is seen, leave the channel.

(3) If confirmation is seen then leave the channel if the ID is not correct, otherwise lock onto the signalling and do not unmute until told to do so.

(4) If confirmation signalling stops, or no signalling is seen on the channel, look for subaudible and unmute.

To understand the significance of the net effect of these procedures, consider two cases: (1) when the channel is not immediately reassigned and (2) when it is immediately reassigned.

It is only possible for the radio to enter late onto the drop channel signalling 100 msec following the point when the drop channel message was sent to the site controller. So if the channel is not being reassigned, a late entering radio will see the additional dotting being transmitted and will know to drop off the channel. On the other hand, if the system is loaded (e.g, call requests are queued in the site controller) the channel immediately gets assigned to the first group in the queue. A radio that attempts to enter late into the call just dropped will see the confirmation message with the group of the next call starting to take place and will know to drop off the channel.

The bottom line is that dropping a channel in a loaded system requires only 100 msec of signalling and only one message to the site controller. Radios that happen to enter late into the call being dropped detect that fact because of the radio's ability to look for the drop channel signalling and the confirmation signalling simultaneously.

For an increase in the radio price, the PST radio manufacturer may program additional of these "features" into radios. The typical prior way to do this is to burn a unique PROM or an EEPROM at the factory. One disadvantage of this approach is the expense of uniquely programming each radio before it leaves the factory--and it is inefficient to upgrade a radio should a customer subsequently desire additional features.

However, the exemplary embodiment permits one to eliminate factory programming costs. Since each radio is programmed in the field (e.g., groups, systems, etc.) by the customer, features should be programmed into the radio at that time. The problem is how to control the programming task sufficiently to make sure the customer only programs purchased features.

Every shipment of radios that goes to a customer will have a sheet of paper which lists a set of Programming Codes and Physical IDs (one pair for each radio). Each Programming Code is an encryption of a "feature enable bitmap" and the Physical ID of the radio.

When a customer programs a radio he/she must do two things. First, he/she programs the radio. To do this, he/she selects the Programming Code representative of the features he/she has purchased for the radio and enters it in the Radio Programmer. He/she then programs the radio using the Radio Programmer while the Programming Code prevents him/her from programming disabled features. Second, the user enters the radio onto the system database via the system manager. The radio's Physical ID must be specified in order to get the radio into the database.

Anytime the Radio Programmer writes data into a radio, it sets a "Just Programmed" bit inside the radio's personality. Whenever a radio is turned on, it checks that bit. If set, the radio will use its Physical ID to request a Logical ID from the site controller before allowing its user to communicate on the trunked system. The site controller will go to the system manager data base to determine the Logical ID to be assigned to the radio. Notice that if a customer tries using the same Programming ID for programming different radios he/she will end up with the same Logical ID in each radio which means unique identification capability is lost. This is the same consequence suffered if a customer copied the PROM used in existing systems.

The result is that one has the same level of protection,--while avoiding the need to program radios in the factory. Adding features to a radio involves issuing an updated Programming ID, ambiguities in programming radios are eliminated (e.g., in existing systems a radio could be programmed to do something it was not enabled to do . . . so when a customer programs it and it doesn't work he/she cannot tell whether the radio is erroneously programmed or the feature is disabled), and no special software is written in the mobiles . . . just the Radio Programmer. This last benefit is nice since fixing a software bug relative to feature enable/disable would involve changing code in just a few computers rather than for all radios in the field.

Detailed descriptions of the signalling protocols and formats involved in many different types of call origination sequences are summarized below:

I. Radio Origination, Logical ID Acquisition Sequence

A. The CC transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec frame in the following frame format:

Dotting=16 bits

Barker=16 bits (e.g., 11 bits Barker code plus 5 bits dotting preamble)

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. When mobile power is turned on, it receives a site ID message from the Control Channel (CC) in the following format:

MT-A=3 bits (e.g.,111)

MT-B=3 bits (e.g.,111)

MT-D=5 bits (e.g.,010dd)

Channel=5 bits

Priority=3 bits

Homesite=1 bit

Failsoft=2 bits

Site ID=6 bits

BCH code=12 bits

Delay (dd in MT-D) specifies the maximum number of control channel slots before a control channel responds to an inbound transmission. Channel specifies the channel number for the active control channel. Priority prohibits mobiles with lower priority from transmitting on the inbound control channel. Home site bit specifies whether the site ID is the home (=0) or adjacent (=1) ID.

C. If desired, and if priority allows, mobile optionally transmits a login request on the control channel in synchronism with the received control channel messages. The frame form is as follows:

Dotting=152 bits

Barker Code (repeated three times)=48 bits (including filler)

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The login message is coded as follows:

MT-A=3 bits

MT-B=3 bits

MT-C=2 bits

Physical ID=20 bits

Logical ID=14 bits

BCH Code=12 bits

    ______________________________________                                         MT-C definition:                                                                             00 ==>   Requesting programming                                                01 ==>   Requesting Logical ID                                                 10 ==>   Logging into the site                                                          (Logical ID)                                                          11 ==>   Undefined                                               ______________________________________                                    

PURPOSE

These commands allow a radio which roams into a system the ability to request programming information. They also allow a radio which is already programmed for a particular system to request a logical ID for temporary usage on the system. Finally, if the need to track key radios between sites ever becomes necessary, the cc=10 message is used to notify the site that the radio has just moved to another site.

II. Radio Call Sequence - Radio Origination, Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a group call transmits a group channel assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=108 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The group call request message is coded as follows:

MT-A=3 bits

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=3 bits

1/2 Logical ID=8 MSBs or LSBs

Channel=5 bits

Transmission Trunked=1 bit

Group ID=11 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=32 (16) bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code=4 bits

Undefined=1 bit

Message/Transmission Trunking=1 bit

Logical ID=14 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. All non-slotted message formats are:

Dotting=128 bits

Data Block #3=128 bits

Data Block #2=128 bits

Data Block #1=128 bits

Date Block #0=128 bits

Data Block #3, #2, #1 and #0 are identical--except for a two bit block count--(each block is repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker Code = 16 bits                                                          Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT Code=4 bits

Block Count=2 bits

Logical ID=14 bits

BCH Code=12 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

III. Radio Call Sequence-Radio Origination, Individual Call

A. Control transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an individual call transmits an assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The individual call request message is in two messages and is coded as follows:

(First Message)

MT-A Code=3 bits

MT-B=3 bits

MT-D=5 bits

Priority=3 bits

Logical ID (caller)=14 bits

BCH Code=12 bits

(Second Message)

MT-A Code=3 bits

MT-B Code=3 bits

MT-D Code=5 bits

MT-E Code=3 bits

Logical ID=14 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair with the logical ID of the called unit sent in the first message and the logical ID or the caller unit sent in the second. Coding is as follows:

MT-A Code=3 bits

MT-B=3 bits

Undefined=2 bits

Channel=5 bits

Qualifier=1 bit

Logical ID=14 bits

BCH Code=12 bits

D. Both calling (last logical ID) and called mobile switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=16 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages:

MT Code=4 bits

Unused bit=1 bit

Message/Transmission Trunking=1 bit

Logical ID=14 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends a non-slotted unkey message. All non-slotted message formats are:

Dotting=128 bits

Data #3=128 bits

Data #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #2, #1, and #0 are identical (repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT Code=4 bits

Block count=2 bits

Logical ID=14 bits

BCH Code=12 bits

IV. Radio Call Sequence-Radio Origination, Emergency Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate an emergency group call transmits an assignment request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The emergency group call request message is coded as follows:

MT-A Code=3 bits

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

C. The control channel responds with two channel-assignment messages that are coded as follows:

MT-A Code=3 bits

1/2 Logical ID=8 MSBs or LSBs

Channel=5 bits

Transmission trunked=1 bit

Group ID=11 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The emergency group call confirmation message is coded as follows:

MT Code=4 bits

Message/Transmission Trunking=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH=12 bits

E. Originating mobile receives confirmation message and transmits 384 bits of dotting, then audio.

F. Working channel receives dotting and transmits two unit-keyed/unmute messages.

MT Code=4 bits

Unused bit=1 bit

Message/Transmission Trunking=1 bit

Logical ID=14 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels receive subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Transmitting mobile unkeys and sends two non-slotted unkey messages. All non-slotted message formats are:

Dotting=128 bits

Data #3=128 bits

Data #2=128 bits

Data #1=128 bits

Data #0=128 bits

Data #3, #1, #1, and #0 are identical (repeated four times) and each has the following format:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte l (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     .                                                                              .                                                                              .                                                                              Byte 3 = 8 bits                                                                Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

The unkey message is coded as follows:

MT Code=4 bits

Block Count=2 bits

Logical ID=14 bits

BCH Code=12 bits

V. Radio Call Sequence-Radio Origination, Status Call

A. Control channel transmits a continuous steam of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a status call transmits a status request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The status request message is coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits

MT-C Code=2 bits

Status=6 bits

Logical ID=14 bits

BCH Code=12 bits

C. The control channel responds with a status page message that is coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits

MT-C Code=2 bits

Status=6 bits

Logical ID=14 bits

BCH Code=12 bits

D. Called mobile transmits a control channel status message that is coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits

MT-C Code=2 bits

Status=6 bits

Logical ID=14 bits

BCH Code=12 bits

E. Control channel responds with a status acknowledge message that is coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits

MT-c Code=2 bits

Status=6 bits

Logical ID=14 bits

BCH Code=12 bits

Originating mobile receives status message.

VI. Radio Call Sequence-Radio Origination, Special Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Mobile that wishes to originate a special call transmits a special call request on the control channel in synchronism with the received control channel messages. The frame format is as follows:

Dotting=152 bits

Barker (repeated three times)=48 bits

Message=40 bits

Message (inverted)=40 bits

Message=40 bits

The special call request message is coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits

MT-C Code=2 bits

3 bits (unused)

Priority Code=3 bits

Logical ID=14 bits

BCH Code=12 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=3 bits

MT-B=3 bits

MT-C=2 bits

Channel=5 bits

Individual/Group=1 bit

Logical ID=14 bits

Group ID=11 bits

BCH Code=12 bits

D. Mobile switches to the assigned working channel and receives a confirmation message. Slotted working channel messages are transmitted using the following frame.

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call confirmation message is coded as follows:

MT Code=4 bits

Hang/time/Trunked=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

E. Originating mobile receives confirmation message and transmits a multi-block special call message. The message frame (shown below) can have from 1 to 16 blocks.

    ______________________________________                                                 Dotting = 96 bits                                                              Data #3 Block #1 = 128 bits                                                    Data #2 Block #1 = 128 bits                                                    Data #1 Block #1 = 128 bits                                                    Data #0 Block #1 = 128 bits                                                    Data #3 Block #2 = 96 bits                                                     Data #2 Block #2 = 96 bits                                                     . -. -.                                                                ______________________________________                                    

Data #3, #2, #1, #0 are identical in each block (repeated four times). Coding for Block #1 Data is:

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 = (inverted) = 8 bits                                                   Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     . -. -. - Byte 3 = 8 bits                                                      Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

Data in blocks after block #1 do not have Barker code bit but do carry a block number field. If telephone interconnect is required, block #1 data is coded as follows:

Digital/IDs=1 bit

Spare=2 bits

Phone Digit Count=5 bits

Phone Digit #1=4 bits MSD

Phone digit #2=4 bits

Phone Digit #3=4 bits

BCH Code=12 bits

If no interconnect is required, block #1 is coded as

Unused=4 bits

Group/Individual=1 bit

Group/Logical ID=14 bits

BCH Code=12 bits

Subsequent blocks are coded with either one group ID, one logical ID, or five telephone digits as required to satisfy the block #1 counts. The telephone digits are first, then the logical ID's, then the group ID's. Digit coding is one digit per nibble (null=1010). ID coding is

8 Bite=10101010 (8 bits)

Group/Logical ID=14 bits

BCH Code=12 bits

F. Working channel transmits a slotted working channel special call receive bitmap message. Slotted working channel messages are transmitted using the following frame:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

BCH Code=12 bits

The special call receive bit map is coded as follows (use of similar acknowledgment bitmaps is the subject of related commonly assigned copending application No. 056,923 filed June 3, 1987):

    ______________________________________                                         MT Code = 4 bits                                                               Block #1 bit = 1 bit (e.g., OK)                                                Block #2 bit = 1 bit (e.g., OK)                                                Block #3 bit = 1 bit (e.g., 0 = repeat)                                        Block #4 bit = 1 bit                                                           . -. -. - Block #16 bit = 1                                                    BCH Code = 12 bits                                                             ______________________________________                                    

G. Originating mobile receives bitmap message and transmits a multi-block special call message. The message frame (shown below) can have from 1 to 16 blocks.

    ______________________________________                                                 Dotting = 96 bits                                                              Data #3 Block #1 = 128 bits                                                    Data #2 Block #1 = 128 bits                                                    Data #1 Block #1 = 128 bits                                                    Data #0 Block #1 = 128 bits                                                    Data #3 Block #2 = 96 bits                                                     Data  2 Block #2 = 96 bits                                                     . -. -.                                                                ______________________________________                                    

Data #3, #2, #1, #0 are identical in each block (repeated four times). Coding for block #1 is

    ______________________________________                                                  Dotting = 16 bits                                                              Barker = 16 bits                                                               Byte 1 = 8 bits                                                                Byte 1 (inverted) = 8 bits                                                     Byte 1 = 8 bits                                                                Byte 2 = 8 bits                                                                Byte 2 (inverted) = 8 bits                                                     . -. -. - Byte 3 = 8 bits                                                      Byte 4 = 8 bits                                                                Byte 4 (inverted) = 8 bits                                                     Byte 4 = 8 bits                                                       ______________________________________                                    

Data in blocks after block #1 do not have dotting or Barker code.

Only blocks that have a "0" in their bitmap bit are transmitted. For example in step F, block #3 in step E would be the first block retransmitted. If no bit map is received within 100 msec. after steps E or G, all blocks are retransmitted.

Steps F and G are repeated until all blocks are received correctly (BITMAP=1's).

H. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

I. The control channel sends from 0 to 16 channel-assignment two-message pairs as required for the special call. Coding for each message is as follows:

MT-A Code=3 bits

1/2 Logical ID=8 MSBs or 6 LSBs

Channel=5 bits

Transmission Trunked=1 bit

Group ID=11 bits

BCH Code=12 bits

J. All called mobiles go to the assigned working channel and unmute (same as late entry). From this point the working channel messages are the same as a group or individual call.

VII. Radio Call Sequence-Console Origination, Group Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate a group call transmits a group call message to the downlink. The group call message is coded as follows:

MID=1 byte (#0)=8 bits

#bytes=1 byte (#1)=8 bits

Source destination=bytes #2 and #3=16 bits

Not Used=4 bits

MT-A Code=3 bits

Group ID=12 bits

Logical ID=14 bits

Parity=1 byte (#8)=8 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=3 bits

1/2 Logical ID=8 MSBs or LSBs

Channel=5 bits

Transmission or Message Trunking=1 bit

Group ID=11 bits

BCH Code=12 bits

D. All mobiles of the called group switch to assigned working channel and receive a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=16 bits

Barker=16 bits

Message=32 bits

Message (inverted)=32 bits

Message=32 bits

The group call confirmation message is coded as follows:

MT Code=4 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

E. Originating console receives channel-assignment from the downlink and switches audio to the specified channel. Console message is coded as follows:

MID=1 Byte (#0)=8 bits

# Bytes=1 Byte (#1)=8 bits

S/D=Bytes #2, #3=16 bits

Not Used=4 bits

MT Code=3 bits

Logical ID - 14 bits

Not Used=2 bits

GR/L ID=1 bit

Channel=5 bits

Group ID=11 bits

Parity=1 Byte (#9)=8 bits

F. Working channel transmits two unit-keyed/unmute messages:

MT Code=4 bits

Unused=1 bit

Hang-time/trunked=1 bit

Logical ID=14 bits

BCH Code=12 bits

Called mobiles receive unmute message and unmute audio.

G. Active mobiles on other working channels receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID=1 Byte (#0)=8 bits

# Bytes=1 byte=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not used=4 bits

MT Code=4 bits

Not Used=2 bits

Logical ID=14 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

Bytes=Byte #1=8 bits

Source Designation=Bytes #2 & #3=16 bits

MT-A/B/C=9 bits

Drop Cr=1 bit

Not Used=1 bit

Channel=5 bits

Logical ID=14 bits

Parity=Byte #8=8 bits

VIII. Radio Call Sequence-Console Origination, Individual Call

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are sent two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to originate an individual call transmits an individual call message to the downlink. The individual call message is coded as follows:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=2 bits

MT-A Code=3 bits

Logical ID=14 bits

Logical ID=14 bits

Parity=Byte #8=8 bits

C. The control channel responds with a channel-assignment two-message pair. Coding is as follows:

MT-A Code=3 bits

1/2 Logical ID=8 MSBs or LSBs

Channel=5 bits

Transmission or Message Trunked=1 bit

Logical ID=14 bits

BCH Code=12 bits

D. Called mobile switches to assigned working channel and receives a confirmation message. Slotted working channel messages are transmitted using the following frame:

Dotting=16 bits

Barker=16 bits

Message=32 bits

Message inverted=32 bits

Message=32 bits

The individual call confirmation message is coded as follows:

MT Code=4 bits

Hang Time/Trunked=1 bit

Group/Logical ID=1 bit

Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

E. Originating console receives channel-assignment message from the downlink and switches audio to the specified channel. Console message is coded as follows:

MID=Byte #0 (8bits)

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=3 bits

Logical ID=14 bits

Not used=2 bits

GR/L ID=1 bit

Channel=5 bits

Logical ID=14 bits

Parity=Byte #9=8 bits

F. Working channel transmits two unit-keyed/unmute messages.

MT Code=4 bits

Unused=1 bit

Hang-Time/Trunked=1 bit

Logical ID=14 bits

BCH Code=12 bits

Called mobile receives unmute message and unmutes audio.

G. Active Mobiles on other working channels do not receive a subaudible channel assignment message.

H. Control channel transmits channel update message for late entry mobiles.

I. Console sends unkey message that is coded as follows:

MID=Byte #0=8 bits

# Bytes=Byte 1=8 bits

Source Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT Code=4 bites

Not Used=2 bits

Logical ID - 14 bits

Parity=Byte #7=8 bits

J. Working channel transmits 896 to 2816 bits of dotting to drop all mobiles from the channel.

K. Console receives unkey message:

MID=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source Destination=Bytes #2 and #3=16 bits

MT-A/B/C=9 bits

Drop Ch=1 bit

Not Used=1 bit

Channel=5 bites

Logical ID=14 bits

Parity=Byte #8=8 bits

IX. Radio Call Sequence Console Origination, Activate Patch

A. Control channel transmits a continuous stream of control messages which all inactive mobiles receive. The messages are set two messages to a 30-msec. frame that has the following format:

Dotting=16 bits

Barker=16 bits

Dotting=16 bits

Message #1=40 bits

Message #1 (inverted)=40 bits

Message #1=40 bits

Message #2=40 bits

Message #2 (inverted)=40 bits

Message #2=40 bits

B. Console that wishes to establish a patch transmits a patch ID assignment message to the downlink. The patch ID assignment message is variable length depending upon the group and individual ID counts:

MID=29=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3=16 bits

Not Used=4 bits

Group Count=4 bits

Individual Count=4 bits

Logical ID=14 bits

Not Used=10 bits

Logical ID=14 bits

Not Used=10 bits

Logical ID=14 bits

Not Used=11 bits

Group ID=11 bits

Not Used - 13 bits

Group ID=11 bits

Not used=13 bits

Patch ID=11 bits

Parity=8 bits

C. Console receives an acknowledgement of the patch request from the site controller using a special group ID code (1000 0000 0000):

MID=12=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2, #3 =16 bits

Not Used=3 bits

MT-A Code=3 bits

MT-B Code=100=3 bits

Patch ID=11 bits

Group ID=11 bits

Parity=Byte #8=8 bits

D. When console wishes to activate the patch it transmits a patch activate message to the downlink.

MID=27=Byte #0=8 bits

# Bytes=Byte #1=05=8 bits

Source/Destination=Bytes #2, & #3=16 bits

Not Used=4 bits

MT Code=4 bits (1110)

Not Used=5 bits

Patch ID=11 bits

Parity=Byte #7=8 bits

E. The control channel responds with alias ID assignment messages. Group assignment messages are coded:

MT-A Code=3 bits

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Not Used=1 bit

Group ID=11 bits

BCH Code=12 bits

Group alias ID messages are repeated in the control channel background mode and not acknowledged by the mobiles.

Individual alias ID assignment messages are coded as follows:

MT-A Code=3 bits

MT-B Code=3 bits (101)

Alias Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

F. All mobiles receive assignment messages but only individual call mobiles acknowledge message.

MT-A Code=3 bits

MT-B Code=3 bits (110)

Alias Group ID=11 bits

Logical ID=14 bits

BCH Code=12 bits

G. Console receives patch assignment/activate messages to confirm patch. One message for each assignment:

MID=12=Byte #0=8 bits

# Bytes=Byte 1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=4 bits

MT-A Code=3 bits (11)

MT-B Code=3 bits (100)

Patch ID=11 bits

Group ID=12 bits

Parity=Byte #8=8 bits

MID=13=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=1 bit

MT-A Code=3 bits

MT-B Code=3 bits (101)

Patch ID=11 bits

Logical ID=14 bits

Parity=Byte #8=8 bits

H. Console originates a patch call by transmitting a group call message using the patch ID:

MID=24=Byte #0=8 bits

# Bytes=Byte #1=8 bits

Source/Destination=Bytes #2 & #3=16 bits

Not Used=3 bits

MT Code=3 bits (00)

Patch ID=12 bits

Logical ID=14 bits

Parity=Byte #8

I. Control channel transmits group call message. Subsequent steps are same as console originated group call.

While only one exemplary embodiment of this invention has been described in detail, those skilled in the art will recognize that many variations and modifications may be made in this embodiment while still retaining many of its novel features and advantages. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims. 

What is claimed is:
 1. A method for achieving reliable and prompt communications with a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:(1) operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel; (2) operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel; (3) passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment; and (4) operating said first radio unit on said second working channel in response to said stream of signals transmitted by said transmitting step (3).
 2. A method as in claim 1 wherein said passing step (3) comprises:(a) continuously passing subaudible digital signalling over said first working channel; and (b) interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.
 3. A method as in claim 2 wherein said passing step (3) futher includes:(c) subsequent to said passing step (b), indicating said second working channel assignment with said subaudible digital signals.
 4. A method as in claim 3 wherein said indicating step (c) is performed only once.
 5. A method as in claim 1 wherein said passing step (3) is repeated over a preset time period.
 6. A method as in claim 1 further comprising the step of:transmitting digital signals indicating a working channel assignment exactly twice on one of said first and second working channels each time a working channel different from said one channel is assigned, once at a high data rate, and the second time at a lower data rate.
 7. A trunked radio repeater system for achieving reliable and prompt communications, said system having a digital control channel and plural working channels which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:means for operating a first radio unit on a first working channel in response to digital assignment signals passed over said control channel; means for operating a second radio unit on a second working channel in response to further digital assignment signals passed over said control channel; and means for passing a stream of digital signals over said first working channel to said first radio unit indicating said second working channel assignment, means wherein said first-mentioned operating means is also for operating said first radio unit on said second working channel in response to said stream of digital signals passed over said first working channel by said passing means.
 8. A system as in claim 7 wherein said passing means comprises:means for continuously passing subaudible digital signalling over said first working channel; and means for interrupting said subaudible digital signals with at least one burst of high-speed digital signalling embedded within said subaudible signals, said high-speed signalling indicating said second working channel assignment.
 9. A system as in claim 8 wherein said subaudible signalling passing means encodes subaudible digital signals with an indication of said second working channel assignment.
 10. A system as in claim 9 wherein said subaudible signalling passing means encodes said subaudible signals with said second working channel assignment indication only once.
 11. A system as in claim 7 wherein said passing means passes digital signals indicating said second working channel assignment for a preset time period less than the time period said second working channel remains assigned.
 12. A method of priority scanning in a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:(1) assigning a first working channel to a first radio unit by passing digital assignment signals over said control channels; (2) passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; (3) subsequently to said passing step (2), passing digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and (4) retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said signals passed by said passing step (3) and/or by said passing step (4).
 13. A method as in claim 12 wherein said passing step (3) includes passing a subaudible digital signalling frame in which every two bits are inverted and retransmitted.
 14. A method as in claim 12 wherein:said passing step (3) includes reserving time slots within said 150 bits per second digital signalling for high-speed data bursts; and said passing step (2) includes filling at least one of said reserved time slots with high-speed digital signalling.
 15. A trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said system including:means for assigning a first working channel to a radio unit by passing digital assignment signals over said control channel; means for passing a burst of digital signals substantially at 9600 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; means cooperating with said first-mentioned passing means for passing subaudible digital signals substantially at 150 bits per second over all assigned ones of said plural working channels indicating the assignment of said first working channel; and means for retuning a further radio unit from a working channel assigned thereto to said first working channel in response to said digital signal burst and/or subaudible digital signals passed over said working channel assigned thereto.
 16. A system as in claim 15 wherein said subaudible signal passing means includes means for transmitting a subaudible digital signalling frame in which every two-bits are inverted.
 17. A system as in claim 15 wherein:said subaudible signal passing means includes means for reserving at least one time slot within said 150 bits per second subaudible digital signalling for high-speed data bursts; and said digital signal burst passing means includes means for filling at least one of said reserved time slots with high-speed digital signalling.
 18. A method of communicating control signals over active working channels of a trunked radio repeater system having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, said method including the steps of:(1) receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said working channel at a rate which is higher than the rate of said subaudible signalling, said subaudible signalling defining a frame structure; (2) receiving high-speed data bursts embedded within said subaudible digital signalling at predetermined positions within said signalling frame structure; and (3) muting audio receiver circuitry during the times said high-speed data bursts are received.
 19. A method as in claim 18 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.
 20. A method as in claim 19 wherein said subaudible signalling frame structure has the following format:(a) 2-bit times reserved for said high-speed data bursts; (b) two additional bits; (c) an idle frame bit which is set whenever no radio units are on the working channel; (d) an 11-bit group identification field; (e) 2-bit times reserved for said high-speed data bursts; (f) two additional bits; (g) a further idle frame bit which is set whenever no radio units are on the working channel; (h) a further 11-bit group identification field; (i) 2-bit times reserved for said high-speed data bursts; (j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and (k) a 12-bit error-checking field.
 21. In a trunked radio repeater system of the type having a digital control channel and plural working channels, which working channels are temporarily assigned for use of individual radio units specified by digital control signals on the control channel, an improvement comprising:means for receiving subaudible digital signalling over a working channel and substantially simultaneously receiving other signals sent over said active working channel at a rate which is higher than the rate of said subaudible signalling; means for receiving high-speed data bursts embedded within said subaudible digital signals at predetermined positions within said signalling; and means cooperating with said high-speed data bursts receiving means for muting audio receiver circuitry during times said high-speed data bursts are being received.
 22. A system as in claim 21 wherein said subaudible signalling defines a frame structure wherein every two bits are inverted and repeated, and said high-speed data bursts occupy 4-bit time slots within said frame structure.
 23. A system as in claim 22 wherein said subaudible signalling frame structure has the following format:(a) 2-bit times reserved for said high-speed data bursts; (b) two additional bits; (c) an idle frame bit which is set whenever no radio units are on the working channels; (d) an 11-bit group identification field; (e) 2-bit times reserved for said high-speed data bursts; (f) two additional bits; (g) a further idle frame bit which is set whenever no radio units are on the working channel; (h) a further 11-bit group identification field; (i) 2-bit times reserved for said high-speed data bursts; (j) 2-bit times reserved for high-speed confirmation signals from radio units on the working channel; and (k) a 12-bit error-checking field.
 24. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:monitoring digital signals passed over an assigned working channel after operation of a radio unit has been established thereon, said digital signals being passed over said working channel first substantially at 9600 bits per second and subsequently at a subaudible data rate, and dropping operation of a radio unit from said working channel in response to a detected change in said digital signals.
 25. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:means for monitoring digital signals transmitted over an assigned working channel after operation of a radio unit has been established thereon, said digital signals including a subaudible signal stream having 9600 bits per second high-speed digital signal bursts embedded therein; and means for dropping operation of a radio unit from said working channel in response to said subaudible signal stream and/or said high-speed signal bursts.
 26. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:requesting assignment of a working channel by passing digital request signals from a first radio unit to a control side over said control channel; assigning a specific working channel to said first radio unit and to at least a second radio unit by passing digital assignment signals over said control channel; passing a digital handshake signal over said assigned working channel from said first radio unit to said control site; substantially simultaneously with said passing step, passing a confirmation message from said control site to said second radio unit; and releasing said first and second units for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful exchange of said handshake signal.
 27. A method as in claim 26 wherein said digital assignment signals include a channel assignment message pair, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.
 28. A method as in claim 26 wherein all said digital signals are transmitted at substantially 9600 bits per second.
 29. A method as in claim 26 wherein said control signals, request signals and handshake signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.
 30. A method as in claim 26 further comprising the steps of:passing digital, new channel assignment updated messages over the assigned working channel first at a high data rate and subsequently at a lower subaudible data rate; and monitoring said new channel assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.
 31. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel; assigning a specific working channel to said first radio unit and to at least one second radio unit by passing digital assignment signals from said control site to said radio units over said control channel; confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site; passing a digital handshake signal over said assigned working channel between said first radio unit and said control site substantially simultaneously with said confirming step; and releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said handshake signal.
 32. A method as in claim 31 wherein said second unit is enabled to communicate by receipt of said release signals.
 33. A method as in claim 31 wherein said assigning step includes the step of passing a channel assignment message pair over said control channel, one message in said pair identifying said first radio unit, the other message in said pair identifying said second radio unit.
 34. A method as in claim 31 wherein said request signals, assignment signals, acknowledgement signals, digital handshake signals and release signals are all digitally encoded and transmitted at substantially 9600 bits per second over said control channel and assigned working channel, respectively.
 35. A method as in claim 31 wherein'said control signals, said request signals and said confirmation signals each include triple data redundancy, and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.
 36. A method as in claim 31 further comprising the steps of:transmitting digital new channel assignment update messages on the assigned working channel first at substantially 9600 bits per second and subsequently at a much lower, subaudible data rate, and monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.
 37. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:A. sending, from a first radio unit to a control site over the control channel, request signals includinga. string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message includingi. 3 bits identifying the message type, ii. 11 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data; B. sending, from said control site to said first radio unit over said control channel, assignment signals includinga. predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits includingi. 3 bits identifying the message type, ii. 8 bits providing one-half the identity of the calling unit; iii. 5 bits identifying the assigned working channel, iv. one bit identifying whether transmission trunking is in effect, v. 11 bits identifying the called group or individual unit, and vi. 12 bits of BCH error detection and correction data; C. sending, from said control site over the assigned working channel, confirmation signals includinga. a predetermined b. a confirmation message of 32 bits repeated three times includingi. 4 bits identifying the type of message, ii. 1 bit identifying whether transmission trunked operation is in effect, iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, and v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data; D. sending, from said first unit over the assigned working channel, digital acknowledgement signals includinga. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called group or individual unit, and E. in response to step D(a) and prior to step D(b), sending, from said control site over the assigned working channel, digital release signals includinga. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages includingi. 4 bits identifying the type of message, ii. 1 bit identifying whether trunked operation is in effect, iii. 14 bits identifying the calling unit, and iv. 12 bits identifying the calling unit, and
 38. A method as in claim 37 further comprising the step of:subsequent to step E, sending digital signals over said assigned working channel including new channel assignment messages first substantially at 9600 bits per second and subsequently at a subaudible data rate, said digital signals being monitored by each unit residing on that working channel for higher priority calls directed to that unit or a predefined group including such unit.
 39. A method as in claim 38 further comprising the step of:subsequent to step E, sending a channel assignment update message over said control channel from the control site to permit late entry of additional called units to the assigned working channel.
 40. A method as in claim 39 further comprising the step of:at the conclusion of communication by said first unit, the transmission from the first unit over said assigned working channel of an unkeyed message includinga. a strin gof dotting bits, b. a block of 128 bits repeated four times, each data block being of the formi. a string of dotting bits, ii. a predetermined word sync code, iii. four 8 bit bytes, each transmitted with triple redundancy and defining a 32 bit message having 4 bits identifying the type of message, a 2-bit block count, 14 bits identifying the calling unit and 12 bits of BCH error detection and correction data.
 41. A method as in claim 40 further comprising the step of:responsive to said unkeyed message, sending, from said control site over said assigned working channel a substantially elongated string of dotting bits, and all units then assigned to the working channel dropping therefrom and reverting to the control channel in response to said elongated string of dotting bits on the working channel.
 42. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel; means for assigning a specific working channel to said first radio unit and to at least a further radio unit by passing digital assignment signals over said control channel; means for confirming the operation of said first radio unit on said assigned working channel by passing a string of synchronization signals from said first radio unit to said control site over said working channel; means for confirming said working channel assignment to said further radio unit by passing digital confirmation message over said assigned working channel from said control site to said further radio unit at substantially the same time said string of synchronization signals are being passed from said first radio unit to said control site; and means for releasing at least one of said first and further unit for communication over said assigned working channel by sending digital release signals on said assigned working channel in response to successful receipt of said synchronization signals.
 43. A system as in claim 42 further comprising:means for passing new channel assignment message over the assigned working channel, and means for monitoring said update messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.
 44. A system as in claim 44 wherein said assigning means includes means for passing a channel assignment two-message pair over said control channel, the first message in said pair identifying said first radio unit, the second message in said pair identifying said second radio unit.
 45. A system as in claim 42 wherein both messages in said pair partially identify said first radio unit.
 46. A system as in claim 44 wherein said passing means includes means for passing at least one high-speed data burst in addition to subaudible digital signals over said assigned working channel both indicating assignment of another working channel.
 47. A system as in claim 42 further comprising:means for sending, from the calling unit on the assigned working channel, a digital unkey message, and means responsive to said unkey message, for sending, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assignment.
 48. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:means for requesting assignment of a working channel by passing digital request signals from a first radio unit to a control site over said control channel; means for assigning a specific working channel to said first radio unit and to at least one further radio unit by passing digital assignment signals from said control site to said radio units over said control channel; means for confirming said assignment by sending digital confirmation signals on said assigned working channel from said control site; means for acknowledging successful receipt of said assignment signals by at least one of said units sending digital acknowledgement signals to said control site over said assigned working channel in response to said confirmation signals; and means for releasing at least one of said units for communication by sending digital release signals over said assigned working channel in response to said acknowledgement signals.
 49. A system as in claim 48 wherein said request signals, assignment signals, acknowledgement signals and release signals are all digitally encoded and transmitted at substantially 9600 bps over said control channel and assigned working channel, respectively.
 50. A system as in claim 48 wherein:said control signals, said request signals and said confirmation signals each include triple data redundancy, and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.
 51. A system for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said system comprising:A. means for sending, from a first radio unit to a control site over the control channel, request signals includinga. a string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message includingi. 4 bits identifying the message type, ii. 11 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data; B. means for sending, from said control site to said first radio unit over said control channel, assignment signals includinga. a predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits includingi. 3 bits identifying the message type, ii. 8 bits providing one-half the identity of the calling unit; iii. 5 bits identifying the assigned working channel, iv. 1 bit identifying whether transmission trunking is in effect, v. 11 bits identifying the called group or individual unit, and vi. 12 bits of BCH error detection and correction data; C. means for sending, from said control site over the assigned working channel, confirmation signals includinga. a predetermined word sync code, and b. a confirmation message of 32 bits repeated three times includingi. 4 bits identifying the type of message, ii. 1 bit identifying whether transmission trunked operation is in effect, iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data; D. means for sending, from said first unit over the assigned working channel, digital acknowledgement signals includinga. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called group or individual unit, and E. means operative in response to said longer string of dotting bits for sending, from said control site over the assigned working channel, digital release signals includinga. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit messages includingi. 4 bits identifying the type of message, ii. 1 bit identifying whether trunked operation is in effect, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data.
 52. A method for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said method comprising the steps of:requesting assignment of a working channel; monitoring digital working channel assignment signals passed over said control channel issued in response to said requested assignment; confirming said channel assignment by passing confirmation digital signals over said assigned working channel between said control site and a first of said radio units and by passing different confirmation signals between said control site and a second of said radio units; and releasing said assigned working channel for communication by sending digital release signals over said assigned working channel in response to successful exchange of said confirmation signals.
 53. A method as in claim 52 wherein said release signals and said different confirmation signals are sent from said control site, and said first-released confirmation signals are sent from said first radio unit.
 54. A method as in claim 52 wherein all said digital signals are transmitted at substantially 9600 bits per second.
 55. A method as in claim 52 wherein said confirmation and release signals include triple data redundancy and said assignment signals include six-fold redundancy of data representing the called party and the assigned channel.
 56. A method as in claim 52 further comprising the steps of:passing new channel assignment messages on the assigned working channel, and monitoring said assignment messages in each radio unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.
 57. A method as in claim 56 wherein said passing step includes:transmitting each new channel assignment message over said working channel at substantially 9600 bits per second; and subsequently transmitting each new channel assignment message at a subaudible rate.
 58. A system of apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use by individual radio units specified through digital control signals on the control channel, said system comprising:means for requesting assignment of a working channel; means for assigning a specific working channel by passing digital assignment signals over said control channel; means for confirming said channel assignment by passing confirmation digital signalling over said assigned working channel between said control site and a first of said radio units and passing different confirmation signals between said control site and a second of said radio units; and means for releasing said assigned working channel for communication thereover by sending digital release signals on said assigned working channel in response to successful exchange of said confirmation signals.
 59. A system as in claim 58 further comprising:means for transmitting subaudible new channel assignment messages on the assigned working channel, and means for monitoring said subaudible assignment messages in each unit and responsively dropping off the assigned working channel if a higher priority incoming message is directed thereto.
 60. A system as in claim 59 wherein said confirming step includes:passing each new channel assignment message over said working channel at substantially 9600 bits per second; and subsequently passing each new channel assignment message only once at a subaudible rate.
 61. A system as in claim 58 further comprising:means for subsequently transmitting digital channel assignment late entry messages on the control channel to permit the late entry of a called unit to an ongoing working channel communication.
 62. A system as in claim 58 further comprising:means for sending, from the calling unit on the assigned working channel, a digital unkey message, and means responsive to said unkey message, for sending, from said control site on said assigned working channel, a digital drop signal which causes all units to drop off the then-assigned working channel thus freeing it for immediate further assignment.
 63. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:means for requesting assignment of a working channel by passing digital request signals to a control site over said control channel; means for detecting digital working channel assignment signals over said control channel; means for passing digital synchronization signals over said assigned working channel, means for detecting digital release signals on said assigned working channel after successfully passing said synchronization signals and for unmuting communication over the assigned working channel in response to said detected release signals; means for detecting new channel assignment digital signals on the assigned working channel, said assignment signals including one of (1) a high-speed burst embedded in a subaudible digital signalling frame on said working channel, and (2) subaudible signals on said working channel, subsequent to said high-speed burst and part of said subaudible signalling frame, and means for responsively dropping off the assigned working channel if a higher priority incoming message is detected.
 64. A system as in claim 63 further comprising:means for sending, from the calling unit on the assigned working channel, a digital unkey message in response to release of a PTT switch, and means responsive to a digital drop signal received on the assigned working channel for dropping off the then-assigned working channel thus freeing it for immediate further assignment.
 65. A radio unit for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said radio unit comprising:A. means for sending, to a control site over the control channel, request signals includinga. a string of dotting bits to establish bit sync, b. a predetermined word sync code repeated three times, and c. a request message of 40 bits repeated three times, each message includingi. 3 bits identifying the message type and type of requested communication, ii. 1 bits identifying the called unit, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data; B. means for detecting on said control channel, assignment signals includinga. a predetermined word sync code, b. a pair of assignment messages, each being repeated three times and having 40 bits includingi. 3 bits identifying the message type, ii. 8 bits providing one-half the identity of the calling unit; working channel, iv. one bit identifying whether transmission trunking operation is in effect, v. 11 bits identifying the called group or individual unit, and vi. 12 bits of BCH error detection and correction data; C. means for detecting on the assigned working channel, confirmation signals includinga. a predetermined word sync code, and b. a confirmation message of 32 bits repeated three times includingi. 4 bits identifying the type of message, ii. 1 bit identifying whether transmission trunked operation is in effect, iii. 1 bit identifying whether a group or individual call is being made, iv. 11 bits identifying the called group or individual unit, v. 14 bits identifying the calling unit, and vi. 12 bits of BCH error detection and correction data; D. means for sending, over the assigned working channel, digital acknowledgement signals includinga. a relatively longer string of dotting bits, followed by b. communications over the working channel to the called group or individual unit, and E. means for detecting on the assigned working channel, digital release signals includinga. a predetermined word sync code, and b. two unit-keyed/unmute messages, each repeated three times and being 32 bit message of the formi. 4 bit identifying the type of message, ii. 2 bits providing a block count, iii. 14 bits identifying the calling unit, and iv. 12 bits of BCH error detection and correction data.
 66. Apparatus for achieving reliable and prompt communication within a trunked radio repeater system having a digital control channel and plural working channels, which working channels are assigned for temporary use of individual radio units specified by digital control signals on the control channel, said apparatus comprising:transceiver means at a central site for transmitting and receiving radio frequency signals over said control channel and over said working channels; and means at said central site operatively coupled to said transceiver means for continually transmitting subaudible signals over an assigned working channel while the working channel remains asigned, said subaudible signals including a predetermined digital data field which is changed to a new value each time a new assignment of that working channel is made, said new value indicating that the working channel has been reassigned. 